long covid research articles

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• Review Article
• Published: 13 January 2023

Long COVID: major findings, mechanisms and recommendations

• Hannah E. Davis   ORCID: orcid.org/0000-0002-1245-2034 1 ,
• Lisa McCorkell   ORCID: orcid.org/0000-0002-3261-6737 2 ,
• Julia Moore Vogel   ORCID: orcid.org/0000-0002-4902-3540 3 &
• Eric J. Topol   ORCID: orcid.org/0000-0002-1478-4729 3  

Nature Reviews Microbiology volume  21 ,  pages 133–146 ( 2023 ) Cite this article

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An Author Correction to this article was published on 17 April 2023

This article has been updated

Long COVID is an often debilitating illness that occurs in at least 10% of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. More than 200 symptoms have been identified with impacts on multiple organ systems. At least 65 million individuals worldwide are estimated to have long COVID, with cases increasing daily. Biomedical research has made substantial progress in identifying various pathophysiological changes and risk factors and in characterizing the illness; further, similarities with other viral-onset illnesses such as myalgic encephalomyelitis/chronic fatigue syndrome and postural orthostatic tachycardia syndrome have laid the groundwork for research in the field. In this Review, we explore the current literature and highlight key findings, the overlap with other conditions, the variable onset of symptoms, long COVID in children and the impact of vaccinations. Although these key findings are critical to understanding long COVID, current diagnostic and treatment options are insufficient, and clinical trials must be prioritized that address leading hypotheses. Additionally, to strengthen long COVID research, future studies must account for biases and SARS-CoV-2 testing issues, build on viral-onset research, be inclusive of marginalized populations and meaningfully engage patients throughout the research process.

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Introduction.

Long COVID (sometimes referred to as ‘post-acute sequelae of COVID-19’) is a multisystemic condition comprising often severe symptoms that follow a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. At least 65 million individuals around the world have long COVID, based on a conservative estimated incidence of 10% of infected people and more than 651 million documented COVID-19 cases worldwide 1 ; the number is likely much higher due to many undocumented cases. The incidence is estimated at 10–30% of non-hospitalized cases, 50–70% of hospitalized cases 2 , 3 and 10–12% of vaccinated cases 4 , 5 . Long COVID is associated with all ages and acute phase disease severities, with the highest percentage of diagnoses between the ages of 36 and 50 years, and most long COVID cases are in non-hospitalized patients with a mild acute illness 6 , as this population represents the majority of overall COVID-19 cases. There are many research challenges, as outlined in this Review, and many open questions, particularly relating to pathophysiology, effective treatments and risk factors.

Hundreds of biomedical findings have been documented, with many patients experiencing dozens of symptoms across multiple organ systems 7 (Fig.  1 ). Long COVID encompasses multiple adverse outcomes, with common new-onset conditions including cardiovascular, thrombotic and cerebrovascular disease 8 , type 2 diabetes 9 , myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) 10 , 11 and dysautonomia, especially postural orthostatic tachycardia syndrome (POTS) 12 (Fig.  2 ). Symptoms can last for years 13 , and particularly in cases of new-onset ME/CFS and dysautonomia are expected to be lifelong 14 . With significant proportions of individuals with long COVID unable to return to work 7 , the scale of newly disabled individuals is contributing to labour shortages 15 . There are currently no validated effective treatments.

The impacts of long COVID on numerous organs with a wide variety of pathology are shown. The presentation of pathologies is often overlapping, which can exacerbate management challenges. MCAS, mast cell activation syndrome; ME/CFS, myalgic encephalomyelitis/chronic fatigue syndrome; POTS, postural orthostatic tachycardia syndrome.

Because diagnosis-specific data on large populations with long COVID are sparse, outcomes from general infections are included and a large proportion of medical conditions are expected to result from long COVID, although the precise proportion cannot be determined. One year after the initial infection, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections increased the risk of cardiac arrest, death, diabetes, heart failure, pulmonary embolism and stroke, as studied with use of US Department of Veterans Affairs databases. Additionally, there is clear increased risk of developing myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and dysautonomia. Six months after breakthrough infection, increased risks were observed for cardiovascular conditions, coagulation and haematological conditions, death, fatigue, neurological conditions and pulmonary conditions in the same cohort. The hazard ratio is the ratio of how often an event occurs in one group relative to another; in this case people who have had COVID-19 compared with those who have not. Data sources are as follows: diabetes 9 , cardiovascular outcomes 8 , dysautonomia 12 , 201 , ME/CFS 10 , 202 and breakthrough infections 4 .

There are likely multiple, potentially overlapping, causes of long COVID. Several hypotheses for its pathogenesis have been suggested, including persisting reservoirs of SARS-CoV-2 in tissues 16 , 17 ; immune dysregulation 17 , 18 , 19 , 20 with or without reactivation of underlying pathogens, including herpesviruses such as Epstein–Barr virus (EBV) and human herpesvirus 6 (HHV-6) among others 17 , 18 , 21 , 22 ; impacts of SARS-CoV-2 on the microbiota, including the virome 17 , 23 , 24 , 25 ; autoimmunity 17 , 26 , 27 , 28 and priming of the immune system from molecular mimicry 17 ; microvascular blood clotting with endothelial dysfunction 17 , 29 , 30 , 31 ; and dysfunctional signalling in the brainstem and/or vagus nerve 17 , 32 (Fig.  3 ). Mechanistic studies are generally at an early stage, and although work that builds on existing research from postviral illnesses such as ME/CFS has advanced some theories, many questions remain and are a priority to address. Risk factors potentially include female sex, type 2 diabetes, EBV reactivation, the presence of specific autoantibodies 27 , connective tissue disorders 33 , attention deficit hyperactivity disorder, chronic urticaria and allergic rhinitis 34 , although a third of people with long COVID have no identified pre-existing conditions 6 . A higher prevalence of long Covid has been reported in certain ethnicities, including people with Hispanic or Latino heritage 35 . Socio-economic risk factors include lower income and an inability to adequately rest in the early weeks after developing COVID-19 (refs. 36 , 37 ). Before the emergence of SARS-CoV-2, multiple viral and bacterial infections were known to cause postinfectious illnesses such as ME/CFS 17 , 38 , and there are indications that long COVID shares their mechanistic and phenotypic characteristics 17 , 39 . Further, dysautonomia has been observed in other postviral illnesses and is frequently observed in long COVID 7 .

There are several hypothesized mechanisms for long COVID pathogenesis, including immune dysregulation, microbiota disruption, autoimmunity, clotting and endothelial abnormality, and dysfunctional neurological signalling. EBV, Epstein–Barr virus; HHV-6, human herpesvirus 6; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

In this Review, we explore the current knowledge base of long COVID as well as misconceptions surrounding long COVID and areas where additional research is needed. Because most patients with long COVID were not hospitalized for their initial SARS-CoV-2 infection 6 , we focus on research that includes patients with mild acute COVID-19 (meaning not hospitalized and without evidence of respiratory disease). Most of the studies we discuss refer to adults, except for those in Box  1 .

Box 1 Long COVID in children

Long COVID impacts children of all ages. One study found that fatigue, headache, dizziness, dyspnoea, chest pain, dysosmia, dysgeusia, reduced appetite, concentration difficulties, memory issues, mental exhaustion, physical exhaustion and sleep issues were more common in individuals with long COVID aged 15–19 years compared with controls of the same age 203 . A nationwide study in Denmark comparing children with a positive PCR test result with control individuals found that the former had a higher chance of reporting at least one symptom lasting more than 2 months 204 . Similarly to adults with long COVID, children with long COVID experience fatigue, postexertional malaise, cognitive dysfunction, memory loss, headaches, orthostatic intolerance, sleep difficulty and shortness of breath 204 , 205 . Liver injury has been recorded in children who were not hospitalized during acute severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections 206 , and although rare, children who had COVID-19 have increased risks of acute pulmonary embolism, myocarditis and cardiomyopathy, venous thromboembolic events, acute and unspecified renal failure, and type 1 diabetes 207 . Infants born to women who had COVID-19 during pregnancy were more likely to receive a neurodevelopmental diagnosis in the first year after delivery 208 . A paediatric long COVID centre’s experience treating patients suggests that adolescents with a moderate to severe form of long COVID have features consistent with myalgic encephalomyelitis/chronic fatigue syndrome 205 . Children experiencing long COVID have hypometabolism in the brain similar to the patterns found in adults with long COVID 209 . Long-term pulmonary dysfunction is found in children with long COVID and those who have recovered from COVID-19 (ref. 210 ). Children with long COVID were more likely to have had attention deficit hyperactivity disorder, chronic urticaria and allergic rhinitis before being infected 34 .

More research on long COVID in children is needed, although there are difficulties in ensuring a proper control group due to testing issues. Several studies have found that children infected with SARS-CoV-2 are considerably less likely to have a positive PCR test result than adults despite seroconverting weeks later, with up to 90% of cases being missed 189 , 190 . Additionally, children are much less likely to seroconvert and, if they develop antibodies, are more likely to have a waning response months after infection compared with adults 193 .

Major findings

Immunology and virology.

Studies looking at immune dysregulation in individuals with long COVID who had mild acute COVID-19 have found T cell alterations, including exhausted T cells 18 , reduced CD4 + and CD8 + effector memory cell numbers 18 , 19 and elevated PD1 expression on central memory cells, persisting for at least 13 months 19 . Studies have also reported highly activated innate immune cells, a lack of naive T and B cells and elevated expression of type I and type III interferons (interferon-β (IFNβ) and IFNλ1), persisting for at least 8 months 20 . A comprehensive study comparing patients with long COVID with uninfected individuals and infected individuals without long COVID found increases in the numbers of non-classical monocytes, activated B cells, double-negative B cells, and IL-4- and IL-6-secreting CD4 + T cells and decreases in the numbers of conventional dendritic cells and exhausted T cells and low cortisol levels in individuals with long COVID at a median of 14 months after infection 18 . The expansion of cytotoxic T cells has been found to be associated with the gastrointestinal presentation of long COVID 27 . Additional studies have found elevated levels of cytokines, particularly IL-1β, IL-6, TNF and IP10 (refs. 40 , 41 ), and a recent preprint has reported persistent elevation of the level of CCL11, which is associated with cognitive dysfunction 42 . It remains to be seen whether the pattern of cytokines in ME/CFS, where the levels of certain cytokines are elevated in the first 2–3 years of illness but decrease over time without a corresponding decrease in symptoms 43 , is similar in long COVID.

Multiple studies have found elevated levels of autoantibodies in long COVID 27 , including autoantibodies to ACE2 (ref. 28 ) (the receptor for SARS-CoV-2 entry), β 2 -adrenoceptor, muscarinic M2 receptor, angiotensin II AT 1 receptor and the angiotensin 1–7 MAS receptor 26 . High levels of other autoantibodies have been found in some patients with COVID-19 more generally, including autoantibodies that target the tissue (such as connective tissue, extracellular matrix components, vascular endothelium, coagulation factors and platelets), organ systems (including the lung, central nervous system, skin and gastrointestinal tract), immunomodulatory proteins (cytokines, chemokines, complement components and cell-surface proteins) 44 . A major comprehensive study, however, did not find autoantibodies to be a major component of long COVID 18 .

Reactivated viruses, including EBV and HHV-6, have been found in patients with long COVID 18 , 21 , 22 , 27 (and have been identified in ME/CFS 45 ), and lead to mitochondrial fragmentation and severely affect energy metabolism 46 . A recent preprint has reported that EBV reactivation is associated with fatigue and neurocognitive dysfunction in patients with long COVID 22 .

Several studies have shown low or no SARS-CoV-2 antibody production and other insufficient immune responses in the acute stage of COVID-19 to be predictive of long COVID at 6–7 months, in both hospitalized patients and non-hospitalized patients 47 , 48 . These insufficient immune responses include a low baseline level of IgG 48 , low levels of receptor-binding domain and spike-specific memory B cells, low levels of nucleocapsid IgG 49 and low peaks of spike-specific IgG 47 . In a recent preprint, low or absent CD4 + T cell and CD8 + T cell responses were noted in patients with severe long COVID 49 , and a separate study found lower levels of CD8 + T cells expressing CD107a and a decline in nucleocapsid-specific interferon-γ-producing CD8 + T cells in patients with long COVID compared with infected controls without long COVID 50 . High levels of autoantibodies in long COVID have been found to be inversely correlated with protective COVID-19 antibodies, suggesting that patients with high autoantibody levels may be more likely to have breakthrough infections 27 . SARS-CoV-2 viral rebound in the gut, possibly resulting from viral persistence, has also been associated with lower levels and slower production of receptor-binding domain IgA and IgG antibodies 51 . There are major differences in antibody creation, seroreversion and antibody titre levels across the sexes, with women being less likely to seroconvert, being more likely to serorevert and having lower antibody levels overall 52 , 53 , even affecting antibody waning after vaccination 54 .

Several reports have pointed towards possible viral persistence as a driver of long COVID symptoms; viral proteins and/or RNA has been found in the reproductive system, cardiovascular system, brain, muscles, eyes, lymph nodes, appendix, breast tissue, hepatic tissue, lung tissue, plasma, stool and urine 55 , 56 , 57 , 58 , 59 , 60 . In one study, circulating SARS-CoV-2 spike antigen was found in 60% of a cohort of 37 patients with long COVID up to 12 months after diagnosis compared with 0% of 26 SARS-CoV-2-infected individuals, likely implying a reservoir of active virus or components of the virus 16 . Indeed, multiple reports following gastrointestinal biopsies have indicated the presence of virus, suggestive of a persistent reservoir in some patients 58 , 61 .

Vascular issues and organ damage

Although COVID-19 was initially recognized as a respiratory illness, SARS-CoV-2 has capability to damage many organ systems. The damage that has been demonstrated across diverse tissues has predominantly been attributed to immune-mediated response and inflammation, rather than direct infection of cells by the virus. Circulatory system disruption includes endothelial dysfunction and subsequent downstream effects, and increased risks of deep vein thrombosis, pulmonary embolism and bleeding events 29 , 30 , 62 . Microclots detected in both acute COVID-19 and long COVID contribute to thrombosis 63 and are an attractive diagnostic and therapeutic target. Long-term changes to the size and stiffness of blood cells have also been found in long COVID, with the potential to affect oxygen delivery 64 . A long-lasting reduction in vascular density, specifically affecting small capillaries, was found in patients with long COVID compared with controls, 18 months after infection 65 . A study finding elevated levels of vascular transformation blood biomarkers in long COVID also found that the angiogenesis markers ANG1 and P-selectin both had high sensitivity and specificity for predicting long COVID status 66 .

An analysis of the US Department of Veterans Affairs databases (VA data) including more than 150,000 individuals 1 year after SARS-CoV-2 infection indicated a significantly increased risk of a variety of cardiovascular diseases, including heart failure, dysrhythmias and stroke, independent of the severity of initial COVID-19 presentation 8 (Fig.  2 ). Cardiac MRI studies revealed cardiac impairment in 78% of 100 individuals who had a prior COVID-19 episode (investigated an average of 71 days after infection 67 ) and in 58% of participants with long COVID (studied 12 months after infection 68 ), reinforcing the durability of cardiac abnormalities.

Multiple studies have revealed multi-organ damage associated with COVID-19. One prospective study of low-risk individuals, looking at the heart, lungs, liver, kidneys, pancreas and spleen, noted that 70% of 201 patients had damage to at least one organ and 29% had multi-organ damage 69 . In a 1-year follow-up study, conducted by the same research group with 536 participants, the study authors found that 59% had single-organ damage and 27% multi-organ damage 70 . A dedicated kidney study of VA data including more than 89,000 individuals who had COVID-19 noted an increased risk of numerous adverse kidney outcomes 71 . Another VA data analysis, including more than 181,000 individuals who had COVID-19, found that infection also increases the risk of type 2 diabetes 9 (Fig.  2 ). The organ damage experienced by patients with long COVID appears durable, and long-term effects remain unknown.

Neurological and cognitive systems

Neurological and cognitive symptoms are a major feature of long COVID, including sensorimotor symptoms, memory loss, cognitive impairment, paresthesia, dizziness and balance issues, sensitivity to light and noise, loss of (or phantom) smell or taste, and autonomic dysfunction, often impacting activities of daily living 7 , 32 . Audiovestibular manifestations of long COVID include tinnitus, hearing loss and vertigo 7 , 72 .

In a meta-analysis, fatigue was found in 32% and cognitive impairment was found in 22% of patients with COVID-19 at 12 weeks after infection 3 . Cognitive impairments in long COVID are debilitating, at the same magnitude as intoxication at the UK drink driving limit or 10 years of cognitive ageing 73 , and may increase over time, with one study finding occurrence in 16% of patients at 2 months after infection and 26% of patients at 12 months after infection 74 . Activation of the kynurenine pathway, particularly the presence of the metabolites quinolinic acid, 3-hydroxyanthranilic acid and kynurenine, has been identified in long COVID, and is associated with cognitive impairment 74 . Cognitive impairment has also been found in individuals who recovered from COVID-19 (ref.  75 ), and at higher rates when objective versus subjective measures were used 3 , suggesting that a subset of those with cognitive impairment may not recognize and/or report their impairment. Cognitive impairment is a feature that manifests itself independently of mental health conditions such as anxiety and depression 74 , 76 , and occurs at similar rates in hospitalized and non-hospitalized patients 74 , 76 . A report of more than 1.3 million people who had COVID-19 showed mental health conditions such as anxiety and depression returned to normal over time, but increased risks of cognitive impairment (brain fog), seizures, dementia, psychosis and other neurocognitive conditions persisted for at least 2 years 77 .

Possible mechanisms for these neuropathologies include neuroinflammation, damage to blood vessels by coagulopathy and endothelial dysfunction, and injury to neurons 32 . Studies have found Alzheimer disease-like signalling in patients with long COVID 78 , peptides that self-assemble into amyloid clumps which are toxic to neurons 79 , widespread neuroinflammation 80 , brain and brainstem hypometabolism correlated with specific symptoms 81 , 82 and abnormal cerebrospinal fluid findings in non-hospitalized individuals with long COVID along with an association between younger age and a delayed onset of neurological symptoms 83 . Multilineage cellular dysregulation and myelin loss were reported in a recent preprint in patients with long COVID who had mild infections, with microglial reactivity similar to that seen in chemotherapy, known as ‘chemo-brain’ 42 . A study from the UK Biobank, including brain imaging in the same patients before and after COVID-19 as well as control individuals, showed a reduction in grey matter thickness in the orbitofrontal cortex and parahippocampal gyrus (markers of tissue damage in areas connected to the primary olfactory cortex), an overall reduction in brain size and greater cognitive decline in patients after COVID-19 compared with controls, even in non-hospitalized patients. Although that study looked at individuals with COVID-19 compared with controls, not specifically long COVID, it may have an implication for the cognitive component of long COVID 84 . Abnormal levels of mitochondrial proteins as well as SARS-CoV-2 spike and nucleocapsid proteins have been found in the central nervous system 85 . Tetrahydrobiopterin deficiencies and oxidative stress are found in long COVID as well 86 .

In the eyes, corneal small nerve fibre loss and increased dendritic cell density have been found in long COVID 87 , 88 , as well as significantly altered pupillary light responses 89 and impaired retinal microcirculation 90 . SARS-CoV-2 can infect and replicate in retinal 59 and brain 91 organoids. Other manifestations of long COVID include retinal haemorrhages, cotton wool spots and retinal vein occlusion 92 .

Mouse models of mild SARS-CoV-2 infection demonstrated microglial reactivity and elevated levels of CCL11, which is associated with cognitive dysfunction and impaired neurogenesis 42 . Hamster models exhibited an ongoing inflammatory state, involving T cell and myeloid activation, production of pro-inflammatory cytokines and an interferon response that was correlated with anxiety and depression-like behaviours in the hamsters, with similar transcriptional signatures found in the tissue of humans who had recovered from COVID-19 (ref. 93 ). Infected non-human primates with mild illness showed neuroinflammation, neuronal injury and apoptosis, brain microhaemorrhages, and chronic hypoxaemia and brain hypoxia 94 .

Recent reports indicate low blood cortisol levels in patients with long COVID as compared with control individuals, more than 1 year into symptom duration 18 , 27 . Low cortisol production by the adrenal gland should be compensated by an increase in adrenocorticotropic hormone (ACTH) production by the pituitary gland, but this was not the case, supporting hypothalamus–pituitary–adrenal axis dysfunction 18 . This may also reflect an underlying neuroinflammatory process. Low cortisol levels have previously been documented in individuals with ME/CFS.

ME/CFS, dysautonomia and related conditions

ME/CFS is a multisystem neuroimmune illness with onset often following a viral or bacterial infection. Criteria include a “substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities” for at least 6 months, accompanied by a profound fatigue that is not alleviated by rest, along with postexertional malaise, unrefreshing sleep and cognitive impairment or orthostatic intolerance (or both) 95 . Up to 75% of people with ME/CFS cannot work full-time and 25% have severe ME/CFS, which often means they are bed-bound, have extreme sensitivity to sensory input and are dependent on others for care 96 . There is a vast collection of biomedical findings in ME/CFS 97 , 98 , although these are not well known to researchers and clinicians in other fields.

Many researchers have commented on the similarity between ME/CFS and long COVID 99 ; around half of individuals with long COVID are estimated to meet the criteria for ME/CFS 10 , 11 , 29 , 100 , and in studies where the cardinal ME/CFS symptom of postexertional malaise is measured, a majority of individuals with long COVID report experiencing postexertional malaise 7 , 100 . A study of orthostatic stress in individuals with long COVID and individuals with ME/CFS found similar haemodynamic, symptomatic and cognitive abnormalities in both groups compared with healthy individuals 101 . Importantly, it is not surprising that ME/CFS should stem from SARS-CoV-2 infection as 27.1% of SARS-CoV infection survivors in one study met the criteria for ME/CFS diagnosis 4 years after onset 102 . A wide range of pathogens cause ME/CFS onset, including EBV, Coxiella burnetii (which causes Q fever), Ross River virus and West Nile virus 38 .

Consistent abnormal findings in ME/CFS include diminished natural killer cell function, T cell exhaustion and other T cell abnormalities, mitochondrial dysfunction, and vascular and endothelial abnormalities, including deformed red blood cells and reduced blood volume. Other abnormalities include exercise intolerance, impaired oxygen consumption and a reduced anaerobic threshold, and abnormal metabolic profiles, including altered usage of fatty acids and amino acids. Altered neurological functions have also been observed, including neuroinflammation, reduced cerebral blood flow, brainstem abnormalities and elevated ventricular lactate level, as well as abnormal eye and vision findings. Reactivated herpesviruses (including EBV, HHV-6, HHV-7 and human cytomegalovirus) are also associated with ME/CFS 97 , 98 , 103 , 104 .

Many of these findings have been observed in long COVID studies in both adults and children (Box  1 ). Long COVID research has found mitochondrial dysfunction including loss of mitochondrial membrane potential 105 and possible dysfunctional mitochondrial metabolism 106 , altered fatty acid metabolism and dysfunctional mitochondrion-dependent lipid catabolism consistent with mitochondrial dysfunction in exercise intolerance 107 , redox imbalance 108 , and exercise intolerance and impaired oxygen extraction 100 , 109 , 110 . Studies have also found endothelial dysfunction 29 , cerebral blood flow abnormalities and metabolic changes 81 , 111 , 112 , 113 (even in individuals with long COVID whose POTS symptoms abate 114 ), extensive neuroinflammation 42 , 80 , reactivated herpesviruses 18 , 21 , 27 , deformed red blood cells 64 and many findings discussed elsewhere. Microclots and hyperactivated platelets are found not only in individuals with long COVID but also in individuals with ME/CFS 115 .

Dysautonomia, particularly POTS, is commonly comorbid with ME/CFS 116 and also often has a viral onset 117 . POTS is associated with G protein-coupled adrenergic receptor and muscarinic acetylcholine receptor autoantibodies, platelet storage pool deficiency, small fibre neuropathy and other neuropathologies 118 . Both POTS and small fibre neuropathy are commonly found in long COVID 111 , 119 , with one study finding POTS in 67% of a cohort with long COVID 120 .

Mast cell activation syndrome is also commonly comorbid with ME/CFS. The number and severity of mast cell activation syndrome symptoms substantially increased in patients with long COVID compared with pre-COVID and control individuals 121 , with histamine receptor antagonists resulting in improvements in the majority of patients 19 .

Other conditions that are commonly comorbid with ME/CFS include connective tissue disorders including Ehlers–Danlos syndrome and hypermobility, neuro-orthopaedic spinal and skull conditions, and endometriosis 33 , 122 , 123 . Evidence is indicating these conditions may be comorbid with long COVID as well. The overlap of postviral conditions with these conditions should be explored further.

Reproductive system

Impacts on the reproductive system are often reported in long COVID, although little research has been done to document the extent of the impact and sex-specific pathophysiology. Menstrual alterations are more likely to occur in women and people who menstruate with long COVID than in women and people who menstruate with no history of COVID and those who had COVID-19 but not long COVID 124 . Menstruation and the week before menstruation have been identified by patients as triggers for relapses of long COVID symptoms 7 . Declined ovarian reserve and reproductive endocrine disorder have been observed in people with COVID-19 (ref. 125 ), and initial theories suggest that SARS-CoV-2 infection affects ovary hormone production and/or the endometrial response due to the abundance of ACE2 receptors on ovarian and endometrial tissue 126 . Individuals with both COVID-19 and menstrual changes were more likely to experience fatigue, headache, body ache and pain, and shortness of breath than those who did not have menstrual changes, and the most common menstrual changes were irregular menstruation, increased premenstrual symptoms and infrequent menstruation 127 .

Research on ME/CFS shows associations between ME/CFS and premenstrual dysphoric disorder, polycystic ovarian syndrome, menstrual cycle abnormalities, ovarian cysts, early menopause and endometriosis 128 , 129 , 130 . Pregnancy, postpartum changes, perimenopause and menstrual cycle fluctuations affect ME/CFS and influence metabolic and immune system changes 129 . Long COVID research should focus on these relationships to better understand the pathophysiology.

Viral persistence in the penile tissue has been documented, as has an increased risk of erectile dysfunction, likely resulting from endothelial dysfunction 131 . In one study, impairments to sperm count, semen volume, motility, sperm morphology and sperm concentration were reported in individuals with long COVID compared with control individuals, and were correlated with elevated levels of cytokines and the presence of caspase 8, caspase 9 and caspase 3 in seminal fluid 132 .

Respiratory system

Respiratory conditions are a common phenotype in long COVID, and in one study occurred twice as often in COVID-19 survivors as in the general population 2 . Shortness of breath and cough are the most common respiratory symptoms, and persisted for at least 7 months in 40% and 20% of patients with long COVID, respectively 7 . Several imaging studies that included non-hospitalized individuals with long COVID demonstrated pulmonary abnormalities including in air trapping and lung perfusion 133 , 134 . An immunological and proteomic study of patients 3–6 months after infection indicated apoptosis and epithelial damage in the airway but not in blood samples 135 . Further immunological characterization comparing individuals with long COVID with individuals who had recovered from COVID-19 noted a correlation between decreased lung function, systemic inflammation and SARS-CoV-2-specific T cells 136 .

Gastrointestinal system

Long COVID gastrointestinal symptoms include nausea, abdominal pain, loss of appetite, heartburn and constipation 137 . The gut microbiota composition is significantly altered in patients with COVID-19 (ref. 23 ), and gut microbiota dysbiosis is also a key component of ME/CFS 138 . Higher levels of Ruminococcus gnavus and Bacteroides vulgatus and lower levels of Faecalibacterium prausnitzii have been found in people with long COVID compared with non-COVID-19 controls (from before the pandemic), with gut dysbiosis lasting at least 14 months; low levels of butyrate-producing bacteria are strongly correlated with long COVID at 6 months 24 . Persisting respiratory and neurological symptoms are each associated with specific gut pathogens 24 . Additionally, SARS-CoV-2 RNA is present in stool samples of patients with COVID-19 (ref. 139 ), with one study indicating persistence in the faeces of 12.7% of participants 4 months after diagnosis of COVID-19 and in 3.8% of participants at 7 months after diagnosis 61 . Most patients with long COVID symptoms and inflammatory bowel disease 7 months after infection had antigen persistence in the gut mucosa 140 . Higher levels of fungal translocation, from the gut and/or lung epithelium, have been found in the plasma of patients with long COVID compared with those without long COVID or SARS-CoV-2-negative controls, possibly inducing cytokine production 141 . Transferring gut bacteria from patients with long COVID to healthy mice resulted in lost cognitive functioning and impaired lung defences in the mice, who were partially treated with the commensal probiotic bacterium Bifidobacterium longum 25 .

The onset and time course of symptoms differ across individuals and by symptom type. Neurological symptoms often have a delayed onset of weeks to months: among participants with cognitive symptoms, 43% reported a delayed onset of cognitive symptoms at least 1 month after COVID-19, with the delay associated with younger age 83 . Several neurocognitive symptoms worsen over time and tend to persist longer, whereas gastrointestinal and respiratory symptoms are more likely to resolve 7 , 74 , 142 . Additionally, pain in joints, bones, ears, neck and back are more common at 1 year than at 2 months, as is paresthesia, hair loss, blurry vision and swelling of the legs, hands and feet 143 . Parosmia has an average onset of 3 months after the initial infection 144 ; unlike other neurocognitive symptoms, it often decreases over time 143 .

Few people with long COVID demonstrate full recovery, with one study finding that 85% of patients who had symptoms 2 months after the initial infection reported symptoms 1 year after symptom onset 143 . Future prognosis is uncertain, although diagnoses of ME/CFS and dysautonomia are generally lifelong.

Diagnostic tools and treatments

Although diagnostic tools exist for some components of long COVID (for example, tilt table tests for POTS 145 and MRI scans to detect cardiovascular impairment 68 ), diagnostic tools for long COVID are mostly in development, including imaging to detect microclots 63 , corneal microscopy to identify small fibre neuropathy 87 , new fragmentation of QRS complex on electrocardiograms as indicative of cardiac injury 146 and use of hyperpolarized MRI to detect pulmonary gas exchange abnormalities 147 . On the basis of the tests that are offered as standard care, the results for patients with long COVID are often normal; many providers are unaware of the symptom-specific testing and diagnostic recommendations from the ME/CFS community 148 . Early research into biomarkers suggests that levels of extracellular vesicles 85 and/or immune markers indicating high cytotoxicity 149 could be indicative of long COVID. Intriguingly, dogs can identify individuals with long COVID on the basis of sweat samples 150 . Biomarker research in ME/CFS may also be applicable to long COVID, including electrical impedance blood tests, saliva tests, erythrocyte deformation, sex-specific plasma lipid profiles and variables related to isocapnic buffering 151 , 152 , 153 , 154 . The importance of developing and validating biomarkers that can be used for the diagnosis of long COVID cannot be adequately emphasized — they will not only be helpful in establishing the diagnosis but will also be helpful for objectively defining treatment responses.

Although there are currently no broadly effective treatments for long COVID, treatments for certain components have been effective for subsets of populations (Table  1 ). Many strategies for ME/CFS are effective for individuals with long COVID, including pacing 7 , 37 and symptom-specific pharmacological options (for example, β-blockers for POTS, low-dose naltrexone for neuroinflammation 155 and intravenous immunoglobulin for immune dysfunction) and non-pharmacological options (including increasing salt intake for POTS, cognitive pacing for cognitive dysfunction and elimination diets for gastrointestinal symptoms) 96 . Low-dose naltrexone has been used in many diseases, including ME/CFS 155 , and has also shown promise in treating long COVID 156 . H 1 and H 2 antihistamines, often following protocols for mast cell activation syndrome and particularly involving famotidine, are used to alleviate a wide range of symptoms 19 , 157 , although they are not a cure. Another drug, BC007, potentially addresses autoimmunity by neutralizing G protein-coupled receptor autoantibody levels 158 . Anticoagulant regimens are a promising way to address abnormal clotting 159 ; in one study, resolution of symptoms was seen in all 24 patients receiving triple anticoagulant therapy 31 . Apheresis has also shown promise to alleviate long COVID symptoms; it has been theorized to help remove microclots 160 and has been shown to reduce autoantibodies in ME/CFS 161 . However, it is quite expensive, and its benefits are uncertain. Some supplements have shown promise in treating both long COVID and ME/CFS, including coenzyme Q 10 and d -ribose 162 , and may deserve further study.

Of note, exercise is harmful for patients with long COVID who have ME/CFS or postexertional malaise 110 , 163 and should not be used as a treatment 164 , 165 , 166 ; one study of people with long COVID noted that physical activity worsened the condition of 75% of patients, and less than 1% saw improvement 109 .

Pilot studies and case reports have revealed additional treatment options worth exploring. A case report noted resolution of long COVID following treatment with the antiviral Paxlovid 167 , and a study investigating the treatment of acute COVID-19 with Paxlovid showed a 25% reduction in the incidence of long COVID 168 ; Paxlovid should be investigated further for prevention and treatment of long COVID. A small trial of sulodexide in individuals with endothelial dysfunction saw a reduction in symptom severity 169 . Pilot studies of probiotics indicated potential in alleviating gastrointestinal and non-gastrointestinal symptoms 170 , 171 . Two patients with long COVID experienced substantial alleviation of dysautonomia symptoms following stellate ganglion block 172 . An early study noted that Pycnogenol statistically significantly improved physiological measurements (for example, reduction in oxidative stress) and quality of life (indicated by higher Karnofsky Performance Scale Index scores) 173 , 174 , as hypothesized on the basis of success in other clinical studies.

Taken together, the current treatment options are based on small-scale pilot studies in long COVID or what has been effective in other diseases; several additional trials are in progress 175 . There is a wide range of possible treatment options from ME/CFS covering various mechanisms, including improving natural killer cell function, removing autoantibodies, immunosuppressants, antivirals for reactivated herpesviruses, antioxidants, mitochondrial support and mitochondrial energy generation 176 , 177 ; most need to be clinically trialled, which should happen urgently. Many newer treatment options remain underexplored, including anticoagulants and SARS-CoV-2-specific antivirals, and a lack of funding is a significant limitation to robust trials.

Impact of vaccines, variants and reinfections

The impact of vaccination on the incidence of long COVID differs across studies, in part because of differing study methods, time since vaccination and definitions of long COVID. One study indicated no significant difference in the development of long COVID between vaccinated individuals and unvaccinated individuals 178 ; other studies indicate that vaccines provide partial protection, with a reduced risk of long COVID between 15% and 41% 4 , 5 , with long COVID continuing to impact 9% of people with COVID-19.

The different SARS-CoV-2 variants and level of (and time since) vaccination may impact the development of long COVID. The UK’s Office for National Statistics found that long COVID was 50% less common in double-vaccinated participants with Omicron BA.1 than in double-vaccinated participants Delta, but that there was no significant difference between triple-vaccinated participants; it also found long COVID was more common after Omicron BA.2 infection than after BA.1 infection in triple-vaccinated participants, with 9.3% developing long COVID from infection with the BA.2 variant 179 .

The impact of vaccination on long COVID symptoms in people who had already developed long COVID differs among patients, with 16.7% of patients experiencing a relief of symptoms, 21.4% experiencing a worsening of symptoms and the remainder experiencing unchanged symptoms 180 .

Reinfections are increasingly common 181 . The impact of multiple instances of COVID-19, including the rate of long COVID in those who recovered from a first infection but developed long COVID following reinfection, and the impact of reinfection on those with pre-existing long COVID is crucial to understand to inform future policy decisions. Early research shows an increasing risk of long COVID sequelae after the second and third infection, even in double-vaccinated and triple-vaccinated people 182 . Existing literature suggests multiple infections may cause additional harm or susceptibility to the ME/CFS-type presentation 33 , 183 .

There is also early evidence that certain immune responses in people with long COVID, including low levels of protective antibodies and elevated levels of autoantibodies, may suggest an increased susceptibility to reinfection 27 .

Challenges and recommendations

Issues with PCR and antibody testing throughout the pandemic, inaccurate pandemic narratives and widespread lack of postviral knowledge have caused downstream issues and biases in long COVID research and care.

Testing issues

Most patients with COVID-19 from the first waves did not have laboratory-confirmed infection, with PCR tests being difficult to access unless individuals were hospitalized. Only 1–3% of cases to March 2020 were likely detected 184 , and the CDC estimates that only 25% of cases in the USA were reported from February 2020 to September 2021 (ref. 185 ); that percentage has likely decreased with the rise in use of at-home rapid tests.

Although PCR tests are our best tool for detecting SARS-CoV-2 infections, their false negative rates are still high 186 . Further bias is caused by false negative rates being higher in women and adults younger than 40 years 187 , those with a low viral load 188 and children (Box  1 ), with several studies showing 52–90% of cases in children missed by PCR tests 189 , 190 . The high false negative PCR rate results in symptomatic patients with COVID-19, who seek a COVID-19 test but receive a false negative result, being included as a control in many studies. Those who have a positive PCR test result (who are more likely to be included in research) are more likely to be male or have a higher viral load. Additionally, the lack of test accessibility as well as the false negative rates has created a significant barrier to care, as many long COVID clinics require PCR tests for admission.

Similarly, there is a broad misconception that everyone makes and retains SARS-CoV-2 antibodies, and many clinicians and researchers are unaware of the limited utility of antibody tests to determine prior infection. Between 22% and 36% of people infected with SARS-CoV-2 do not seroconvert, and many others lose their antibodies over the first few months, with both non-seroconversion and seroreversion being more likely in women, children and individuals with mild infections 52 , 53 , 191 , 192 , 193 . At 4 and 8 months after infection, 19% and 61% of patients, respectively, who had mild infections and developed antibodies were found to have seroreverted, compared with 2% and 29% of patients, respectively who had severe infections 191 . Still, many clinicians and researchers use antibody tests to include or exclude patients with COVID-19 from control groups.

Furthermore, during periods of test inaccessibility, tests were given on the basis of patients having COVID-19-specific symptoms such as loss of smell and taste, fever, and respiratory symptoms, resulting in a bias towards people with those symptoms.

Misinformation on PCR and antibody tests has resulted in the categorization of patients with long COVID into non-COVID-19 control groups, biasing the research output. Because low or no antibody levels and viral load may be related to long COVID pathophysiology, including a clinically diagnosed cohort will strengthen the research.

Important miscues

The narrative that COVID-19 had only respiratory sequelae led to a delayed realization of the neurological, cardiovascular and other multisystem impacts of COVID-19. Many long COVID clinics and providers still disproportionately focus on respiratory rehabilitation, which results in skewed electronic health record data. Electronic health record data are also more comprehensive for those who were hospitalized with COVID-19 than for those who were in community care, leading to a bias towards the more traditional severe respiratory presentation and less focus on non-hospitalized patients, who tend to have neurological and/or ME/CFS-type presentations.

The narrative that initially mild COVID-19 cases, generally defined as not requiring hospitalization in the acute phase, would not have long-term consequences has also had downstream effects on research. These so-called mild cases that result in long COVID often have an underlying biology different from acute severe cases, but the same types of tests are being used to evaluate patients. This is despite basic tests such as D-dimer, C-reactive protein (CRP) and antinuclear antibody tests and complete blood count being known to often return normal results in patients with long COVID. Tests that return abnormal results in patients with ME/CFS and dysautonomia, such as total immunoglobulin tests, natural killer cell function tests, the tilt table or NASA lean test, the four-point salivary cortisol test, reactivated herpesvirus panels, small fibre neuropathy biopsy, and tests looking for abnormal brain perfusion 96 , should instead be prioritized. Other recurring issues include studies failing to include the full range of symptoms, particularly neurological and reproductive system symptoms, and not asking patients about symptom frequency, severity and disability. Cardinal symptoms such as postexertional malaise are not widely known, and therefore are rarely included in study designs.

Widespread lack of postviral knowledge and misinformation

The widespread lack of knowledge of viral-onset illnesses, especially ME/CFS and dysautonomia, as well as often imperfect coding, prevents these conditions from being identified and documented by clinicians; this means that they are frequently absent from electronic health record data. Further, because ME/CFS and dysautonomia research is not widely known or comprehensively taught in medical schools 194 , long COVID research is often not built on past findings, and tends to repeat old hypotheses. Additionally, long COVID research studies and medical histories tend to document only the risk factors for severe acute COVID-19, which are different from the risk factors for conditions that overlap with long COVID such as ME/CFS and dysautonomia (for example, connective tissue disorders such as Ehlers–Danlos syndrome, prior illnesses such as infectious mononucleosis and mast cell involvement) 33 , 195 , 196 .

Clinicians who are not familiar with ME/CFS and dysautonomia often misdiagnose mental health disorders in patients; four in five patients with POTS receive a diagnosis with a psychiatric or psychological condition before receiving a POTS diagnosis, with only 37% continuing to have the psychiatric or psychological diagnosis once they have received their POTS diagnosis 117 . Researchers who are unfamiliar with ME/CFS and dysautonomia often do not know to use specific validated tools when conducting mental health testing, as anxiety scales often include autonomic symptoms such as tachycardia, and depression scales often include symptoms such as fatigue, both of which overestimate mental health disorder prevalence in these conditions 197 , 198 .

Recommendations

Although research into long COVID has been expansive and has accelerated, the existing research is not enough to improve outcomes for people with long COVID. To ensure an adequate response to the long COVID crisis, we need research that builds on existing knowledge and is inclusive of the patient experience, training and education for the health-care and research workforce, a public communication campaign, and robust policies and funding to support research and care in long COVID.

We need a comprehensive long COVID research agenda that builds on the existing knowledge from ME/CFS, dysautonomia and other viral-onset conditions, including but not limited to brain and brainstem inflammation, appropriate neuroimaging techniques, neuroimmunology, metabolic profiling, impaired endothelial function, mitochondrial fragmentation, antiviral and metabolic phenotypes, hypoperfusion/cerebral blood flow, nanoneedle diagnostic testing, overlaps with connective tissue disorders, autoimmunity and autoantibodies, viral/microbial persistence, intracranial hypertension, hypermobility, craniocervical obstructions, altered T and B cells, metabolomics and proteomics, elevated blood lactate level, herpesvirus reactivations, immune changes in the early versus late postviral years, and changes to the gut microbiota. The mechanisms of and overlaps between long COVID and connective tissue involvement, mast cells and inflammatory conditions such as endometriosis are particularly understudied and should be focused on. Because of the high prevalence of ME/CFS, POTS and other postinfectious illnesses in patients with long COVID, long COVID research should include people who developed ME/CFS and other postinfectious illnesses from a trigger other than SARS-CoV-2 in comparator groups to improve understanding of the onset and pathophysiology of these illnesses 113 . Additionally, there is a known immune exhaustion process that occurs between the second and third year of illness in ME/CFS, with test results for cytokines being different between patients who have been sick for shorter durations (less than 2 years) than for those who have been sick for longer durations 43 . Because of this, studies should implement subanalyses based on the length of time participants have been ill. Because ME/CFS and dysautonomia research is not widely known across the biomedical field, long COVID research should be led by experts from these areas to build on existing research and create new diagnostic and imaging tools.

Robust clinical trials must be a priority moving forward as patients currently have few treatment options. In the absence of validated treatment options, patients and physicians conduct individual experiments, which result in the duplication of efforts without generalizable knowledge and pose undue risks to patients. Robust study design and knowledge sharing must be prioritized by both funding institutions and clinician-researchers.

It is critical that research on long COVID be representative of (or oversample) the populations who had COVID-19 and are developing long COVID at high rates, which is disproportionately people of colour 35 . Medical research has historically under-represented these populations, and over-representation of white and socio-economically privileged patients has been common in long COVID research. Researchers must work within communities of colour, LGBTQ+ communities and low-income communities to build trust and conduct culturally competent studies that will provide insights and treatments for long COVID for marginalized populations.

As a subset of patients will improve over time, and others will have episodic symptoms, care should be taken to incorporate the possibility of alleviation of symptoms into the study design, and care should be taken not to ascribe improvement to a particular cause without proper modelling.

Finally, it is critical that communities of patients with long COVID and associated conditions are meaningfully engaged in long COVID research and clinical trials. The knowledge of those who experience an illness is crucial in identifying proper study design and key research questions and solutions, improving the speed and direction of research.

Training and education of the health-care and research workforce

To prepare the next generation of health-care providers and researchers, medical schools must improve their education on pandemics, viruses and infection-initiated illnesses such as long COVID and ME/CFS, and competency evaluations should include these illnesses. As of 2013, only 6% of medical schools fully cover ME/CFS across the domains of treatment, research and curricula, which has created obstacles to care, accurate diagnosis, research and treatment 194 . To ensure people with long COVID and associated conditions can receive adequate care now, professional societies and government agencies must educate the health-care and research workforce on these illnesses, including the history of and current best practices for ME/CFS to not repeat mistakes of the past, which have worsened patients’ prognoses. The research community has made a misstep in its efforts to treat ME/CFS 199 , and some physicians, poorly educated in the aetiology and pathophysiology of the disorder, still advise patients to pursue harmful interventions such as graded exercise therapy and cognitive behavioural therapy, despite the injury that these interventions cause 200 and the fact that they are explicitly not advised as treatments 163 , 164 , 166 .

Public communications campaign

In addition to providing education on long COVID to the biomedical community, we need a public communications campaign that informs the public about the risks and outcomes of long COVID.

Policies and funding

Finally, we need policies and funding that will sustain long COVID research and enable people with long COVID to receive adequate care and support. For instance, in the USA, the creation of a national institute for complex chronic conditions within the NIH would go a long way in providing a durable funding mechanism and a robust research agenda. Further, we need to create and fund centres of excellence, which would provide inclusive, historically informed and culturally competent care, as well as conduct research and provide medical education to primary care providers. Additionally, research and clinical care do not exist in silos. It is critical to push forward policies that address both the social determinants of health and the social support that is needed for disabled people.

Conclusions

Long COVID is a multisystemic illness encompassing ME/CFS, dysautonomia, impacts on multiple organ systems, and vascular and clotting abnormalities. It has already debilitated millions of individuals worldwide, and that number is continuing to grow. On the basis of more than 2 years of research on long COVID and decades of research on conditions such as ME/CFS, a significant proportion of individuals with long COVID may have lifelong disabilities if no action is taken. Diagnostic and treatment options are currently insufficient, and many clinical trials are urgently needed to rigorously test treatments that address hypothesized underlying biological mechanisms, including viral persistence, neuroinflammation, excessive blood clotting and autoimmunity.

Acknowledgements

We would like to thank the long COVID and associated conditions patient and research community and the entire team at Patient-Led Research Collaborative. E.J.T. was supported by National Center for Advancing Translational Sciences (NCATS) grant UL1TR002550.

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Davis, H.E., McCorkell, L., Vogel, J.M. et al. Long COVID: major findings, mechanisms and recommendations. Nat Rev Microbiol 21 , 133–146 (2023). https://doi.org/10.1038/s41579-022-00846-2

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March 1, 2023

17 min read

Long COVID Now Looks like a Neurological Disease, Helping Doctors to Focus Treatments

The causes of long COVID, which disables millions, may come together in the brain and nervous system

By Stephani Sutherland

Stephanie Shafer

T ara Ghormley has always been an overachiever. She finished at the top of her class in high school, graduated summa cum laude from college and earned top honors in veterinary school. She went on to complete a rigorous training program and build a successful career as a veterinary internal medicine specialist. But in March 2020 she got infected with the SARS-CoV-2 virus—just the 24th case in the small, coastal central California town she lived in at the time, near the site of an early outbreak in the COVID pandemic. “I could have done without being first at this,” she says.

Almost three years after apparently clearing the virus from her body, Ghormley is still suffering. She gets exhausted quickly, her heartbeat suddenly races, and she goes through periods where she can't concentrate or think clearly. Ghormley and her husband, who have relocated to a Los Angeles suburb, once spent their free time visiting their “happiest place on Earth”—Disneyland—but her health prevented that for more than a year. She still spends most of her days off resting in the dark or going to her many doctors' appointments. Her early infection and ongoing symptoms make her one of the first people in the country with “long COVID,” a condition where symptoms persist for at least three months after the infection and can last for years. The syndrome is known by medical professionals as postacute sequelae of COVID-19, or PASC.

People with long COVID have symptoms such as pain, extreme fatigue and “brain fog,” or difficulty concentrating or remembering things. As of March 2023, the syndrome was estimated to affect more than 15 million adults in the U.S., and a 2022 report found that it had forced between two million and four million Americans out of the workforce. Long COVID often arises in otherwise healthy young people, and it can follow even a mild initial infection. The risk appears at least slightly higher in people who were hospitalized for COVID and in older adults (who end up in the hospital more often). Women and those at socioeconomic disadvantage also face higher risk, as do people who smoke, are obese, or have any of an array of health conditions, particularly autoimmune disease. Vaccination appears to reduce the danger but does not entirely prevent long COVID.

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The most common, persistent and disabling symptoms of long COVID are neurological. Some are easily recognized as brain- or nerve-related: many people experience cognitive dysfunction in the form of difficulty with memory, attention, sleep and mood. Others may seem rooted more in the body than the brain, such as pain and postexertional malaise (PEM), a kind of “energy crash” that people experience after even mild exercise. But those, too, result from nerve dysfunction, often in the autonomic nervous system, which directs our bodies to breathe and digest food and generally runs our organs on autopilot. This so-called dysautonomia can lead to dizziness, a racing heart, high or low blood pressure, and gut disturbances, sometimes leaving people unable to work or even function independently.

The SARS-CoV-2 virus is new, but postviral syndromes are not. Research on other viruses, and on neurological damage from the human immunodeficiency virus (HIV) in particular, is guiding work on long COVID. And the recognition that the syndrome may cause its many effects through the brain and the nervous system is beginning to shape approaches to medical treatment. “I now think of COVID as a neurological disease as much as I think of it as a pulmonary disease, and that's definitely true in long COVID,” says William Pittman, a physician at UCLA Health in Los Angeles, who treats Ghormley and many similar patients.

Although 15 million current U.S. sufferers is a reasonable estimate of the condition's toll, there are other, more dire assessments. A meta-analysis of 41 studies conducted in 2021 concluded that worldwide, 43 percent of people infected with SARS-CoV-2 may develop long COVID, with about 30 percent—translating to approximately 30 million people—affected in the U.S. Some studies have offered more conservative numbers. A June 2022 survey reported by the U.S. National Center for Health Statistics found that among adults who had had COVID, one in five was experiencing long COVID three months later; the U.K. Office for National Statistics put the estimate at one in 10. Even if only a small share of infections result in long COVID, experts say, they will add up to millions more people affected—and potentially disabled.

Most of the first recognized cases of long COVID were in patients who needed extended respiratory therapy or who had obvious organ damage that caused lasting symptoms. People reporting neurological symptoms were often overlooked or dismissed as traumatized by their initial illness and hospitalization. But as 2020 came to an end, says Helen Lavretsky, a psychiatrist at the University of California, Los Angeles, “we started getting to a place of sorting through what was really going on ... and it became very evident at that time that neuropsychiatric symptoms were quite prevalent,” most commonly fatigue, malaise, brain fog, smell loss and post-traumatic stress disorder, as well as cognitive problems and even psychosis.

Ghormley was in her late 30s and relatively healthy when she caught the virus, but she had underlying conditions—including rheumatoid arthritis and asthma—that put her at risk for severe COVID. She spent several days at home, struggling to breathe, and then she went to the hospital, where her blood pressure soared and her blood glucose dropped precipitously. She mostly recovered from this acute phase within a few weeks, but, she says, “I never really got better.”

Soon after coming home from the hospital, Ghormley developed what her husband called “goldfish brain.” “I'd put something down and have no idea where I put it,” she recalls. “It kept happening over and over. I was thinking, ‘This is getting weird.' My husband said I was not remembering anything. I'd try to talk, and I knew what I wanted to say, but I couldn't think of the word.”

“Everything fell apart for me,” says Tara Ghormley, who has been struggling with long COVID since 2020. Credit: Ewan Burns

She also experienced tremors, dramatic mood swings and painful hypersensitivity to sounds. “My husband opening a paper bag felt like knives stabbing me in the ear,” she recounts. Any exertion—physical or mental—left her exhausted and in pain. The changes were jarring to Ghormley, who prided herself on her sharp mind. “The thing that bothered me the most was that I was really having trouble thinking, speaking, remembering—trying to complete a task and having no idea what it was. Suddenly I had quite profound neurological deficits. Everything fell apart for me at that time. That was horribly traumatic ... it kind of broke me. I didn't feel like me.”

Roots of Dysfunction

As a veterinary internist, Ghormley says, it's her job to problem solve when mysterious symptoms arise, including her own. “I was actively trying to find reasons and find what I could do.” She theorized that some of her neurological symptoms might be the result of thrombotic events, blood clots that can cause ministrokes. Several early studies showed that COVID attacks endothelial cells, which line blood vessels. That can lead to clotting and oxygen deprivation in multiple organs, including the brain. Even subtle disruption of endothelial cells in the brain could contribute to cognitive dysfunction.

One study found that in people with neurological COVID symptoms, the immune system seems to be activated specifically in the central nervous system, creating inflammation. But brain inflammation is probably not caused by the virus infecting that organ directly. Avindra Nath, who has long studied postviral neurological syndromes at the National Institutes of Health, found something similar in an autopsy study of people who died of COVID. “When you look at the COVID brain, you don't actually find [huge amounts of virus, but] we found a lot of immune activation,” he says, particularly around blood vessels. The examinations suggested that immune cells called macrophages had been stirred up. “Macrophages are not that precise in their attack,” Nath says. “They come and start chewing things up; they produce all kinds of free radicals, cytokines. It's almost like blanket bombing—it ends up causing a lot of damage. And they're very hard to shut down, so they persist for a long time. These are the unwelcome guests” that may be causing persistent inflammation in the brain.

Determining which patients have ongoing inflammation could help inform treatments. Early research identified markers that often are elevated in people with the condition, says Troy Torgerson, an immunologist at the Allen Institute in Seattle. Three cell-signaling molecules—tumor necrosis factor alpha, interleukin 6 and interferon beta—stood out in long COVID patients. But this pattern wasn't found in absolutely everyone. “We're trying to sort through long COVID patients and say, ‘This would be a good group to take to trials of an anti-inflammatory drug, whereas this group may need to focus more on rehabilitation,'” Torgerson says. He led a study (currently released as a preprint, without formal scientific review by a journal) in which his team measured proteins from the blood of 55 patients. The researchers found that a subset had persistent inflammation. Among those people, they saw a distinct immune pathway linked to a lasting response to infection. “One subset of patients does appear to have an ongoing response to some virus,” Torgerson says.

Isolated pockets of SARS-CoV-2 or even pieces of viral proteins may remain in the body well after the initial infection and continue to elicit an immune attack. The first solid evidence for “viral persistence” outside the lungs came in 2021 from researchers in Singapore who found viral proteins throughout the gut in five patients who had recovered from COVID as much as six months earlier. A study conducted at the University of California, San Francisco, found evidence for viral particles in the brains of people with long COVID. Scientists collected exosomes, or tiny packets of cellular material, released specifically from cells of the central nervous system. The exosomes contained pieces of viral proteins as well as mitochondrial proteins, which may indicate an immune attack on those vital cellular organelles. Amounts of such suspicious proteins were higher in patients with neuropsychiatric symptoms than in those without them.

The virus could linger in the brain for months, according to research conducted at the nih and reported in Nature in December 2022. The autopsy study of 44 people who died of COVID found rampant inflammation mainly in the respiratory tract, but viral RNA was detected throughout the body, even in the brain, as long as 230 days after infection. Two other studies, both published last year in the Proceedings of the National Academy of Sciences USA , showed evidence that SARS-CoV-2 may infect astrocytes, a type of neural support cell, gaining entrance via neurons in the skin lining the nose.

Researchers are examining inflammatory signals in patients with long COVID in increasingly fine detail. A small study led by Joanna Hellmuth, a neurologist at U.C.S.F., found that patients with cognitive symptoms had immune-related abnormalities in their cerebrospinal fluid, whereas none of the patients without cognitive symptoms did. At the 2022 meeting of the Society for Neuroscience, Hellmuth reported that she had looked at more specific immune markers in people with cognitive symptoms and found that some patients had an elevated level of VEGF-C, a marker of endothelial dysfunction. Higher VEGF-C concentrations are associated with higher levels of immune cells getting into the brain, she says, and “they're not doing their normal function of maintaining the blood-brain barrier; they're distracted and perhaps activated.” Although the studies are small, Hellmuth adds, they reveal “real biological distinctions and inflammation in the brain. This is not a psychological or psychosomatic disorder; this is a neuroimmune disorder.”

What keeps the immune system in attack mode? According to Torgerson, “one option is that you've developed autoimmunity,” in which antibodies produced by the immune system to fight the virus also mark a person's own cells for immune attack. The response to the virus “turns the autoimmunity on, and that doesn't get better even when the virus goes away,” he says. Several studies have found evidence of autoimmune components called autoantibodies that interact with nerve cells in people with long COVID.

Clues about the inflammatory processes at work could point toward treatments for neurological symptoms. “If it's a macrophage-mediated inflammatory process ... intravenous immunoglobulin could make a difference [to] dampen the macrophages,” Nath says. The treatment, referred to as IVIg, contains a cocktail of proteins and antibodies that can mitigate an overactive immune response.

IVIg can also be used to block autoantibodies. And a therapy called rituximab that targets antibody-producing B cells provides “a time-tested therapy for a lot of autoantibody-mediated syndromes,” Nath says. Another strategy is to use corticosteroids to dampen immune activity altogether, although those drugs can be used for only a limited time. “That's a sledgehammer approach, and you can see if it makes a difference. At least it gives you an idea that, yes, it's an immune-mediated phenomenon, and now we need to find a better way to target it,” Nath says.

If the virus does hang around in some form, antiviral medications could potentially clear it, which might help resolve neurological symptoms. That's the hope of scientists running a clinical trial of Paxlovid, Pfizer's antiviral drug for acute COVID.

A Chronic Fatigue Connection?

Postviral syndromes have been documented for more than a century, arising after infection with viruses from HIV to the flu. Epstein-Barr virus, which causes mononucleosis, is one of several viruses linked to a condition called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), which is estimated to affect at least one and a half million people in the U.S. ME/CFS bears striking resemblances to long COVID, with symptoms such as immune system dysregulation, fatigue and cognitive dysfunction. “One of the patterns we see is patients who definitely meet the criteria for ME/CFS. This is something we are seeing and treating all the time” in long COVID patients, Pittman says. ME/CFS can be severe, with some people losing mobility and becoming bedbound.

Credit: Now Medical Studios; Sources: “Neurovascular Injury with Complement Activation and Inflammation in COVID-19,” by Myoung-Hwa Lee et al., in Brain , Vol. 145; July 2022 ( blood vessel reference ); “Olfactory Transmucosal SARS-CoV-2 Invasion as a Port of Central Nervous System Entry in Individuals with COVID-19,” by Jenny Meinhardt et al., in Nature Neuroscience , Vol. 24; February 2021 ( nasal passage reference )

Nath, who also studies ME/CFS, says that “we think mechanistically they are going to be related.” Researchers suspect that ME/CFS, like some cases of long COVID, could be autoimmune in nature, with autoantibodies keeping the immune system activated. ME/CFS has been difficult to study because it often arises long after a mild infection, making it hard to identify a viral trigger. But with long COVID, Nath says, “the advantage is that we know exactly what started the process, and you can catch cases early in the [development of] ME/CFS-like symptoms.” In people who have had ME/CFS for years, “it's done damage, and it's hard to reverse that.” Nath speculates that for long COVID, if doctors could study people early in the illness, they would have a better chance of reversing the process.

Torgerson hopes that researchers will ultimately come to better understand ME/CFS because of COVID. “COVID has been more carefully studied with better technology in the time we've had it than any other infectious disease ever. I think we'll learn things that will be applicable to other inflammatory diseases driven by infection followed by an autoimmune process.”

Team Treatment

Ghormley, after months of illness, sought care at UCLA Health's long COVID clinic, among the country's few comprehensive, multidisciplinary programs for people with this syndrome. Even though her symptoms are rooted in nervous system dysfunction, she needed an array of medical specialists to treat them. The clinic grew out of a program aimed at coordinating care for medically complex COVID patients, says its director Nisha Viswanathan, an internist and primary care physician. In following up with COVID patients after several months, she realized that “we had a group of patients who still had symptoms. There was no understanding around the condition; we were just trying to see what we could offer them.” Viswanathan and others convened a biweekly meeting of UCLA Health doctors in pulmonology, cardiology, neurology, psychiatry and other specialties to discuss individual cases and overall trends.

At UCLA Health, Pittman coordinates Ghormley's treatment. He says the interdisciplinary team is crucial to getting patients the best possible care. “Oftentimes there are so many symptoms,” and some patients have seen multiple specialists before arriving, but not necessarily the right ones. As long COVID primary care providers, he says, “we do the initial testing and get them to the right person.” For Ghormley, that list of providers includes Pittman, along with a neurologist, a pulmonologist, a cardiologist, a psychiatrist, a trauma counselor, a rheumatologist and a gynecologist.

The team approach has also been critical for doctors trying to understand a brand-new disease, Pittman says. “It's been a very interesting journey from knowing almost nothing to knowing a little bit now, and we're learning more every day, every week, every month,” he says. The term “long COVID” “is an umbrella, and I think there are multiple diseases under that umbrella.” Although each long COVID patient is unique, Pittman says, “we start to see patterns developing. And with Ghormley, we saw a pattern of dysautonomia, which we see frequently.”

Dysautonomia impairs the autonomic nervous system, a network of nerves that branch out from the brain or spinal cord and extend through the body, controlling unconscious functions such as heartbeat, breathing, sweating and blood vessel dilation. For Ghormley, like many people with long COVID, dysautonomia takes the form of postural orthostatic tachycardia syndrome, or POTS. The syndrome encompasses a collection of symptoms that include a racing heart rate—particularly on standing—and fatigue, and it can cause bowel and bladder irregularities. POTS can also be a component of the exhaustion that comes with PEM. Although the symptoms may seem to affect the body, they stem from nervous system dysfunction.

Ghormley's dysautonomia led her to see cardiologist Megha Agarwal at a UCLA clinic near her home. Many physicians are not familiar with POTS, but Agarwal is particularly attuned to it, having seen it in some of her patients before COVID hit. “There's dysregulation of the nervous system, and so many things can cause it: some cancer therapies, viruses, autoimmune conditions.” Agarwal recognized POTS in Ghormley in the fall of 2020, when very little was known about long COVID. Now she believes “POTS is really what long-haul COVID is causing” in many patients. Luckily, Agarwal says, there are medical interventions that can help.

Tachycardia—the T in POTS—causes the heartbeat to speed up, contributing to exhaustion and fatigue in addition to stressing the heart itself. Drugs called beta-blockers (for the beta-adrenergic receptors they shut off in the heart) can lower the heart rate and improve symptoms. “When heart rate is controlled, not only does the pump improve,” Agarwal says, “[but people's] energy improves, their fatigue is gone, and sometimes there's better mental clarity.” For some patients like Ghormley, beta-blockers are not enough, so Agarwal adds a medication called ivabradine. “It's a bit off-label, but it's currently being aggressively studied” for POTS. For Ghormley, the combination led to real improvements, “so now she doesn't feel like she ran the Boston Marathon when all she did was sit down and stand up at work or take a shower,” Agarwal says.

Among Ghormley's toughest symptoms is her brain fog, a catchall term for a slew of cognitive problems that make it hard for her to function. For days when Ghormley works, her psychiatrist prescribes Adderall, a stimulant used to treat attention deficit hyperactivity disorder that helps her concentrate and stay focused. That has “helped immensely,” Ghormley says.

Ghormley credits her doctors and Agarwal in particular with doing the detective work to dig into her symptoms. “Nobody knew anything about it, but everyone listened to me,” Ghormley says. Perhaps because she was a professional from a medical field, no one “brushed me aside.”

That's unusual for people with long COVID, many of them women, who are often dismissed by physicians who doubt their complaints are real. “Patients just don't feel heard,” Viswanathan says. “I had a patient who told me everything, and after, I just said, ‘This must be so hard for you. I want you to know that everything you're feeling is real, and I've seen so many patients like you.' And she started crying. She said, ‘No one has told me that. I can't tell you the number of times I was told it was in my head.’”

Credit: Now Medical Studio; Sources: “Postural Orthostatic Tachycardia Syndrome as a Sequela of COVID-19,” by Cameron K. Ormiston et al., in Heart Rhythm , Vol. 19; November 2022; “Long COVID-19 and Postural Orthostatic Tachycardia Syndrome—Is Dysautonomia to Be Blamed?” by Karan R. Chadda et al., in Frontiers in Cardiovascular Medicine ; March 2022 ( references )

In addition to drugs, other types of therapies, including physical therapy, can help improve some symptoms. But people who experience PEM face a particular challenge when using movement therapies. Pittman says the exertion can make these patients feel worse. “We don't want patients to go to not moving at all, but sometimes the type of movement they're doing may be flaring their symptoms.” He notes that often PEM strikes young, previously healthy people who will say, “‘I need to push myself,' and then they go way too far and get worse. Our job is to try to find that middle ground and then make that consistent over time, so they're not getting further deconditioned but they don't have the PEM, which has been shown to set them back.”

The Long Haul

Some patients, Pittman says, “have the expectation that they're going to come in, and within a month they're going to be back to normal. And resetting those expectations can be really challenging. You have to be really empathetic because people's lives have completely changed.” But sometimes patients' quality of life can improve noticeably when they are able to adjust to a new normal. Still, he says, “patients have so many questions, and I can't lead them down a physiological pathway. I can tell them there's neuroinflammation, maybe there's autoimmunity, but we still don't have the answers. Sometimes it's really tough for us to accept and for the patient to accept that we just have to try our best.”

A number of people, Viswanathan says, benefit from reducing various treatments they have accumulated. Some people become so desperate that they will try anything from supplements to off-label medications to untested potions from the Internet. Stopping those sometimes leads to improved symptoms, she says.

Psychological care and support groups can help. Lavretsky adds that “lifestyle choices can play a huge role in improvement,” particularly better sleep habits and the use of breathing exercises to control anxiety. She tells people their bodies can heal themselves if the patients and clinicians find the right tools.

Whether that's true for everyone remains to be seen, Viswanathan says. “We see many patients who have gotten better with time. I have patients whose symptoms have disappeared in the course of a year, or they disappear and occasionally flare up again.” But for some, she says, “it could last many years.”

“We're going to be addressing this for probably decades,” Viswanathan says. “COVID is not going to go away so much as we're just going to get used to living with it, but part of [that] means that people will continue to develop long COVID.”

Vaccination appears to reduce the risk of long COVID. But a study published in May 2022 in Nature Medicine suggests the protection, though real, is not as good as one might hope. The survey of electronic health records from the U.S. Department of Veterans Affairs looked at the relatively small portion of vaccinated people who subsequently became infected. They developed long COVID only 15 percent less often than unvaccinated people. “These patients can have symptoms for one to two years or longer, and so every month you're racking up more patients. Even if it's 15 percent less, the total population of patients is still growing and exploding,” Pittman says. The best way to avoid getting long COVID, experts all agree, is to avoid getting COVID at all.

The syndrome is still mired in a lot of medical uncertainty. Patients might have one or a combination of the problems investigated so far: Long COVID might be caused by viral particles that persist in the brain or other parts of the nervous system. Or it might be an autoimmune disorder that lasts long after the virus has disappeared. Maybe overactive immune cells continue to perturb the nervous system and nearby blood vessels. Fortunately, the increasing ability to recognize specific problems is helping clinicians hone treatments that give patients the best chance of recovery.

Although Ghormley says her care has dramatically improved her symptoms and allowed her to “do some normal things again,” she continues to experience flare-ups that make it impossible for her to work for weeks at a time. One day last year she skipped a dose of her heart medication and made a Target run in the southern California heat. “I got home and basically collapsed in the hallway,” she says, and even months later things were “out of whack. If I try to move around, my legs give out.” Most frustrating—and scary—to Ghormley is the unpredictability of her symptoms. “They have changed so much; some are manageable, some debilitating. One thing will get better, and another thing comes back. I'm always hopeful that it's going to get better, but I just don't know.”

Stephani Sutherland is a neuroscientist and science journalist based in southern California. She wrote about the causes of autoimmune diseases in our September 2021 issue. Follow her on Twitter @SutherlandPhD

Premium Content

• CORONAVIRUS COVERAGE

Is long COVID forever? A new study has clues.

Research hints that those with symptoms may see an improvement after two years—but not everyone.

A new study shows that an abnormally active immune system, a characteristic of long COVID patients, largely subsides two years after the initial infection. The finding offers hope that gradual recovery is possible in some long COVID patients.

One in 10 COVID-19 patients, even those with a mild infection, suffer from a range of symptoms such as fatigue, brain fog, shortness of breath, heart palpitations, and depression. These lingering symptoms collectively called long COVID are now recognized as disability under the Americans with Disabilities Act and can continue for weeks, months, or even years.

While scientists don’t know what causes long COVID, a hyperactive immune system appears to play a major role in persistent symptoms. A new study , published in Nature Communications , now shows that this heightened immune response calms down in long COVID patients 24 months after initial infection.

"It looks like those people's immune systems have largely returned to what we would expect," says Gail Matthews, an infectious disease physician at St Vincent's Hospital, Sydney, who led the study. "And that's really good."

( Why does COVID-19 cause brain fog? Scientists may finally have an answer. )

The study doesn't explain why some long COVID patients did not get better, but this could be due to other underlying health conditions, says Matthews. However, there is evidence of significant improvements in both immunological disturbances and the self-reported health of many patients two years post-COVID.

For Hungry Minds

"It's good news for those with long COVID that there seems to be resolution of not only immune perturbations but also symptoms over time," says Nadia Roan, an immunologist at The J. David Gladstone Institutes at the University of California San Francisco.

The blood markers of long COVID

Scientists at the Kirby Institute, UNSW Sydney, began collecting blood samples from some COVID-19 patients in April 2020 when the first wave of the pandemic hit Australia . These patients regularly self-report their health information and were not vaccinated when they got COVID-19 because the shots were not available in Australia until early 2021 .

In 2022 , scientists found that immune-system molecules related to inflammation remained abnormally high in the blood of patients who felt fatigue, shortness of breath, or chest pain, eight months after infection. The levels of these molecules, called cytokines, should return to normal levels within 30-90 days after recovering from the initial viral infection.

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"It was quite surprising for us to see that even after mild to moderate COVID-19, the inflammatory biomarkers were high up until eight months for long COVID patients," says Chansavath Phetsouphanh, an immunologist at the Kirby Institute, UNSW Sydney. Phetsouphanh led the study that first hinted at a prolonged abnormal immune response in people who had long COVID compared to those who had been infected with SARS-CoV-2 and fully recovered. The biomarkers included molecules that activate immune cells—called T cells—and trigger inflammation.

In their recent study, the scientists followed up on 62 of the long COVID patients for an additional one and a half years and found that biomarkers previously indicating abnormal immune function in their blood samples had largely fallen to normal levels. They found no difference in the blood inflammatory biomarkers between the recovered long COVID patients and those who had never reported any long COVID symptoms.

"It is a well-done study [that] does show that some people can improve immune function over time," says Timothy Henrich, a professor of medicine at the University of California San Francisco.

There are other causes of long COVID

While the Nature Communications study builds on previously identified biomarker signatures that predicted the persistence of long COVID symptoms at eight months, Wolfram Ruf, an immunologist at Johannes Gutenberg University Medical Center in Mainz, Germany, says other immune markers that have also been linked to long COVID were not taken into consideration in this recent study.

For example, the study team didn’t address the overactivation of the "complement" part of the immune system —which protects the body from infection and inflammation—and has been implicated in causing long COVID. Similarly, the team did not assess the abnormal blood clotting or coagulation system , which has also been shown to cause some long COVID symptoms.

( Long COVID can destroy your ability to exercise. Now we know why. )

While 62 percent of the patients in the Australian study also reported improved health during the follow-up period, the rest of the cohort claimed that long COVID was still curbing the quality of life despite an improved inflammation biomarker profile.

"We want to be really careful not to dismiss the fact that some people did not recover," says Matthews. This could be because there are other causes of long COVID ; the persistence of SARS-CoV-2 virus; reactivation of other dormant viruses; the formation of autoantibodies that attack a person’s own organs and tissues; or disturbed gut microbes.

"The immunological basis for long COVID does not explain all the symptoms, and we know that long COVID is a basket of potential different diagnoses," says Matthews.

Other studies involving more patients, such as the National Institutes of Health's RECOVER Initiative , are expected to reveal why symptoms do not resolve years after initial infection in many people with long COVID.

"Immune perturbations are clearly associated with long COVID,” says Roan. "Whether they drive it is still an open question."

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Long COVID Research Is in Its ‘Most Hopeful’ Phase Yet

A phenomenal amount of research on Long COVID—the name for chronic symptoms following a case of COVID-19—has been published over the past three years. But scientific advances have yet to bring relief to people who are already sick, a group estimated to include about 5% of U.S. adults but hard to precisely quantify due to the difficulty of diagnosing people correctly .

Researchers are optimistic that breakthroughs are coming. The U.S. National Institutes of Health (NIH) has launched multiple clinical trials focused on potential therapies , and several recent studies have pointed to biomarkers that may help doctors accurately diagnose—and, hopefully, treat—people with Long COVID.

“In the short history of studying this disease, this is probably the most hopeful moment we’ve ever had,” says Christoph Thaiss, an assistant professor of microbiology at the University of Pennsylvania’s Perelman School of Medicine who co-authored a recent study on Long COVID in Cell .

More From TIME

The search for biomarkers.

Long COVID is currently a disease mostly defined by its symptoms, which range from brain fog and fatigue to headaches and nervous-system dysfunction. There is no single test that can diagnose it—although recent research points to a variety of potential testing methods, from full-body scans to eye exams .

A September study published in Nature was widely heralded as a step toward finally having a blood test to confirm Long COVID. Researchers analyzed blood samples from almost 300 people, some of whom had Long COVID, some of whom had never had COVID-19, and some of whom had it and fully recovered. Long COVID patients tended to have low levels of the stress hormone cortisol, and their blood also often suggested that virus lingered in their bodies—either remnants of the virus that causes COVID-19, or other viruses that had been dormant in the body after prior infections and become reactivated.

Co-author Akiko Iwasaki, an immunobiologist who directs Yale University School of Medicine's Center for Infection and Immunity, says it's unlikely there will ever be a single biomarker for Long COVID, since the disease can look very different from person to person. But when a machine-learning model was trained to pick up on all of those potential signals together, it was able to distinguish the blood of Long COVID patients from the blood of people without the condition with 96% accuracy.

“That doesn’t mean we’re going to have biomarkers next week,” Iwasaki says, “but I think we are moving forward in the right direction.”

Thaiss’ recent study in Cell found another potential biomarker in the blood of Long COVID patients: low levels of the neurotransmitter serotonin, which is largely produced in the gut and involved in numerous bodily functions. Using stool samples, Thaiss and his colleagues found genetic material from the SARS-CoV-2 virus in the gastrointestinal tracts of a subset of Long COVID patients, mirroring results from other studies. They then hypothesized, and used mice to demonstrate, that this stubborn viral material can trigger an immune response that leads to excess inflammation in the body, in turn hampering the gut’s production of serotonin. Inadequate serotonin seems to contribute to a number of neurological symptoms of Long COVID .

The study is important because it “gets us closer to understanding what’s happening” in the bodies of people with Long COVID, says Hannah Davis, one of the leaders of the Patient-Led Research Collaborative for Long COVID.

Low levels of cortisol or serotonin could be useful "signals" for assessing people with Long COVID, but—at least for now—they cannot serve as stand-alone diagnostics, says Dr. Adupa Rao, medical director of the COVID Recovery Clinic at the University of Southern California’s Keck School of Medicine. For one thing, the studies on potential Long COVID biomarkers have been fairly small and must be replicated in larger groups of patients, Rao says. Beyond that, there are plenty of reasons why someone would have low cortisol or serotonin, including non-COVID viral infections, he says.

Will biomarkers lead to treatments?

Still, Maayan Levy, who co-authored the Cell study and is also an assistant professor of microbiology at UPenn, believes serotonin may be a target for Long COVID treatment. Building on their findings in mice, her group is designing a clinical trial to test whether selective serotonin reuptake inhibitors (SSRIs)— a widely prescribed class of antidepressant , including fluoxetine (Prozac) and escitalopram (Lexapro), used to boost serotonin levels in the brain—are effective against Long COVID. She also plans to test whether supplementing tryptophan, an amino acid that the body uses to make serotonin, may be beneficial.

But Davis is skeptical. SSRIs are already so widely used, she says, that “if SSRIs worked, we would know.” And researchers working on ME/CFS, a condition similar enough to Long COVID that many long-haulers meet its diagnostic criteria , have previously warned that tryptophan supplementation may be dangerous for patients . “It would be good to not reinvent the wheel” by re-testing these ideas, Davis says, noting that there is plenty of research on ME/CFS treatment that could inform researchers working on Long COVID.

In her view, there are other, more promising possible treatments in the research pipeline, including antivirals (which could, in theory, destroy remnants of either reactivated viruses or the SARS-CoV-2 virus lingering in the body); drugs that interact with the immune system; and medications that prevent blood clotting .

Rao says he's not sure any drug currently being studied will become a magic bullet, but he’s encouraged by the field’s progress. “My hope is, in the near future, we’ll be able to identify the cause [of Long COVID] and therefore provide more thorough treatment options, rather than treating just the symptoms,” Rao says.

In Davis' view, testing treatments may achieve both goals: “Understanding what drugs help patients," she says, "will also help us understand what’s actually happening in patient bodies.”

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Write to Jamie Ducharme at [email protected]

Long COVID or Post-COVID Conditions

Some people who have been infected with the virus that causes COVID-19 can experience long-term effects from their infection, known as Long COVID or Post-COVID Conditions (PCC). Long COVID is broadly defined as signs, symptoms, and conditions that continue or develop after acute COVID-19 infection. This definition  of Long COVID was developed by the Department of Health and Human Services (HHS) in collaboration with CDC and other partners.

People call Long COVID by many names, including Post-COVID Conditions, long-haul COVID, post-acute COVID-19, long-term effects of COVID, and chronic COVID. The term post-acute sequelae of SARS CoV-2 infection (PASC) is also used to refer to a subset of Long COVID.

What You Need to Know

• Long COVID is a real illness and can result in chronic conditions that require comprehensive care. There are resources available .
• Long COVID can include a wide range of ongoing health problems; these conditions can last weeks, months, or years.
• Long COVID occurs more often in people who had severe COVID-19 illness, but anyone who has been infected with the virus that causes COVID-19 can experience it.
• People who are not vaccinated against COVID-19 and become infected may have a higher risk of developing Long COVID compared to people who have been vaccinated.
• People can be reinfected with SARS-CoV-2, the virus that causes COVID-19, multiple times. Each time a person is infected or reinfected with SARS-CoV-2, they have a risk of developing Long COVID.
• While most people with Long COVID have evidence of infection or COVID-19 illness, in some cases, a person with Long COVID may not have tested positive for the virus or known they were infected.
• CDC and partners are working to understand more about who experiences Long COVID and why, including whether groups disproportionately impacted by COVID-19 are at higher risk.

In July 2021, Long COVID was added as a recognized condition that could result in a disability under the Americans with Disabilities Act (ADA). Learn more: Guidance on “Long COVID” as a Disability Under the ADA .

About Long COVID

Long COVID is a wide range of new, returning, or ongoing health problems that people experience after being infected with the virus that causes COVID-19. Most people with COVID-19 get better within a few days to a few weeks after infection, so at least 4 weeks after infection is the start of when Long COVID could first be identified. Anyone who was infected can experience Long COVID. Most people with Long COVID experienced symptoms days after first learning they had COVID-19, but some people who later experienced Long COVID did not know when they got infected.

There is no test that determines if your symptoms or condition is due to COVID-19. Long COVID is not one illness. Your healthcare provider considers a diagnosis of Long COVID based on your health history, including if you had a diagnosis of COVID-19 either by a positive test or by symptoms or exposure, as well as based on a health examination.

Science behind Long COVID

RECOVER: Researching COVID to Enhance Recovery

People with Long COVID may experience many symptoms.

People with Long COVID can have a wide range of symptoms that can last weeks, months, or even years after infection. Sometimes the symptoms can even go away and come back again. For some people, Long COVID can last weeks, months, or years after COVID-19 illness and can sometimes result in disability.

Long COVID may not affect everyone the same way. People with Long COVID may experience health problems from different types and combinations of symptoms that may emerge, persist, resolve, and reemerge over different lengths of time. Though most patients’ symptoms slowly improve with time, speaking with your healthcare provider about the symptoms you are experiencing after having COVID-19 could help determine if you might have Long COVID.

People who experience Long COVID most commonly report:

General symptoms ( Not a Comprehensive List)

• Tiredness or fatigue that interferes with daily life
• Symptoms that get worse after physical or mental effort (also known as “ post-exertional malaise ”)

Respiratory and heart symptoms

• Difficulty breathing or shortness of breath
• Fast-beating or pounding heart (also known as heart palpitations)

Neurological symptoms

• Difficulty thinking or concentrating (sometimes referred to as “brain fog”)
• Sleep problems
• Dizziness when you stand up (lightheadedness)
• Pins-and-needles feelings
• Change in smell or taste
• Depression or anxiety

Digestive symptoms

• Stomach pain

Other symptoms

• Joint or muscle pain
• Changes in menstrual cycles

Symptoms that are hard to explain and manage

Some people with Long COVID have symptoms that are not explained by tests or easy to manage.

People with Long COVID may develop or continue to have symptoms that are hard to explain and manage. Clinical evaluations and results of routine blood tests, chest X-rays, and electrocardiograms may be normal. The symptoms are similar to those reported by people with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and other poorly understood chronic illnesses that may occur after other infections. People with these unexplained symptoms may be misunderstood by their healthcare providers, which can result in a delay in diagnosis and receiving the appropriate care or treatment.

Review these tips to help prepare for a healthcare provider appointment for Long COVID.

Health conditions

Some people experience new health conditions after COVID-19 illness.

Some people, especially those who had severe COVID-19, experience multiorgan effects or autoimmune conditions with symptoms lasting weeks, months, or even years after COVID-19 illness. Multi-organ effects can involve many body systems, including the heart, lung, kidney, skin, and brain. As a result of these effects, people who have had COVID-19 may be more likely to develop new health conditions such as diabetes, heart conditions, blood clots, or neurological conditions compared with people who have not had COVID-19.

People experiencing any severe illness may develop health problems

People experiencing any severe illness, hospitalization, or treatment may develop problems such as post-intensive care syndrome (PICS).

PICS refers to the health effects that may begin when a person is in an intensive care unit (ICU), and which may persist after a person returns home. These effects can include muscle weakness, problems with thinking and judgment, and symptoms of post-traumatic stress disorder  (PTSD), a long-term reaction to a very stressful event. While PICS is not specific to infection with SARS-CoV-2, it may occur and contribute to the person’s experience of Long COVID. For people who experience PICS following a COVID-19 diagnosis, it is difficult to determine whether these health problems are caused by a severe illness, the virus itself, or a combination of both.

People More Likely to Develop Long COVID

Some people may be more at risk for developing Long COVID.

Researchers are working to understand which people or groups of people are more likely to have Long COVID, and why. Studies have shown that some groups of people may be affected more by Long COVID. These are examples and not a comprehensive list of people or groups who might be more at risk than other groups for developing Long COVID:

• People who have experienced more severe COVID-19 illness, especially those who were hospitalized or needed intensive care.
• People who had underlying health conditions prior to COVID-19.
• People who did not get a COVID-19 vaccine.

Health Inequities May Affect Populations at Risk for Long COVID

Some people are at increased risk of getting sick from COVID-19 because of where they live or work, or because they can’t get health care. Health inequities may put some people from racial or ethnic minority groups and some people with disabilities at greater risk for developing Long COVID. Scientists are researching some of those factors that may place these communities at higher risk of getting infected or developing Long COVID.

Preventing Long COVID

The best way to prevent Long COVID is to protect yourself and others from becoming infected. For people who are eligible, CDC recommends staying up to date on COVID-19 vaccination , along with improving ventilation, getting tested for COVID-19 if needed, and seeking treatment for COVID-19 if eligible. Additional preventative measures include avoiding close contact with people who have a confirmed or suspected COVID-19 illness and washing hands  or using alcohol-based hand sanitizer.

Research suggests that people who get a COVID-19 infection after vaccination are less likely to report Long COVID, compared to people who are unvaccinated.

CDC, other federal agencies, and non-federal partners are working to identify further measures to lessen a person’s risk of developing Long COVID. Learn more about protecting yourself and others from COVID-19 .

Living with Long COVID

Living with Long COVID can be hard, especially when there are no immediate answers or solutions.

People experiencing Long COVID can seek care from a healthcare provider to come up with a personal medical management plan that can help improve their symptoms and quality of life. Review these tips  to help prepare for a healthcare provider appointment for Long COVID. In addition, there are many support groups being organized that can help patients and their caregivers.

Although Long COVID appears to be less common in children and adolescents than in adults, long-term effects after COVID-19 do occur in children and adolescents .

Talk to your doctor if you think you or your child has Long COVID. Learn more: Tips for Talking to Your Healthcare Provider about Post-COVID Conditions

Data for Long COVID

Studies are in progress to better understand Long COVID and how many people experience them.

CDC is using multiple approaches to estimate how many people experience Long COVID. Each approach can provide a piece of the puzzle to give us a better picture of who is experiencing Long COVID. For example, some studies look for the presence of Long COVID based on self-reported symptoms, while others collect symptoms and conditions recorded in medical records. Some studies focus only on people who have been hospitalized, while others include people who were not hospitalized. The estimates for how many people experience Long COVID can be quite different depending on who was included in the study, as well as how and when the study collected information.  Estimates of the proportion of people who had COVID-19 that go on to experience Long COVID can vary.

CDC posts data on Long COVID and provides analyses, the most recent of which can be found on the U.S. Census Bureau’s Household Pulse Survey .

CDC and other federal agencies, as well as academic institutions and research organizations, are working to learn more about the short- and long-term health effects associated with COVID-19 , who gets them and why.

Scientists are also learning more about how new variants could potentially affect Long COVID. We are still learning to what extent certain groups are at higher risk, and if different groups of people tend to experience different types of Long COVID. CDC has several studies that will help us better understand Long COVID and how healthcare providers can treat or support patients with these long-term effects. CDC will continue to share information with healthcare providers to help them evaluate and manage these conditions.

CDC is working to:

• Better identify the most frequent symptoms and diagnoses experienced by patients with Long COVID.
• Better understand how many people are affected by Long COVID, and how often people who are infected with COVID-19 develop Long COVID
• Better understand risk factors and protective factors, including which groups might be more at risk, and if different groups experience different symptoms.
• Help understand how Long COVID limit or restrict people’s daily activity.
• Help identify groups that have been more affected by Long COVID, lack access to care and treatment for Long COVID, or experience stigma.
• Better understand the role vaccination plays in preventing Long COVID.
• Collaborate with professional medical groups to develop and offer clinical guidance and other educational materials for healthcare providers, patients, and the public.

Related Pages

• Caring for People with Post-COVID Conditions
• Preparing for Appointments for Post-COVID Conditions
• Researching COVID to Enhance Recovery
• Guidance on “Long COVID” as a Disability Under the ADA

For Healthcare Professionals

• Post-COVID Conditions: Healthcare Providers

Search for and find historical COVID-19 pages and files. Please note the content on these pages and files is no longer being updated and may be out of date.

• Visit archive.cdc.gov for a historical snapshot of the COVID-19 website, capturing the end of the Federal Public Health Emergency on June 28, 2023.
• Visit the dynamic COVID-19 collection  to search the COVID-19 website as far back as July 30, 2021.

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News Release

Wednesday, May 8, 2024

NIH to open long COVID clinical trials to study sleep disturbances, exercise intolerance, and post exertional malaise

Part of the NIH RECOVER Initiative, trials will test four treatments.

The National Institutes of Health (NIH) will launch clinical trials to investigate potential treatments for long-term symptoms after COVID-19 infection, including sleep disturbances, exercise intolerance and the worsening of symptoms following physical or mental exertion known as post-exertional malaise (PEM). The mid-stage trials, part of NIH’s Researching COVID to Enhance Recovery (RECOVER) Initiative, will join six other RECOVER studies currently enrolling participants across the United States testing treatments to address viral persistence, neurological symptoms, including cognitive dysfunction (like brain fog) and autonomic nervous system dysfunction. The new trials will enroll approximately 1,660 people across 50 study sites to investigate potential treatments for some of the most frequent and burdensome symptoms reported by people suffering from long COVID.

“The group of symptoms these trials will try to alleviate are truly disruptive and devastating for so many people struggling with long COVID,” said Walter J. Koroshetz, M.D., director of NIH’s National Institute of Neurological Disorders and Stroke, and co-lead of the RECOVER Initiative. “When people can’t get reliable sleep, can’t exert themselves and feel sick following tasks that used to be simple, the physical and mental anguish can lead to feelings of utter helplessness. We urgently need to come up with answers to help those struggling with long COVID feel whole again.”

RECOVER-SLEEP clinical trials will soon begin enrolling participants and include:

• A trial to test two drugs (modafinil and solriamfetol) approved by the Food and Drug Administration to treat people who have problems staying awake during the day, known as hypersomnia. These medications are well-known but have not been studied widely in people with long COVID. Participants will be randomly assigned to receive either the active study drug or a placebo control for eight to 10 weeks, depending on the assigned study drug.
• A trial to test potential treatments for complex sleep disturbances due to long COVID, including melatonin, an over-the-counter supplement commonly used to treat people with sleep disorders and general insomnia; and light therapy, which is used to help people reset their sleep cycles. Participants will be randomly assigned to receive either melatonin or a placebo control, and either high-intensity (active) light therapy or low-intensity (placebo) light therapy for eight weeks. 

RECOVER-ENERGIZE clinical trials will soon begin enrolling participants and include:

• A trial to test a program that combines exercise training, strength and flexibility training,  education, and social support, collectively known as personalized cardiopulmonary rehabilitation. The program is designed to help people who experience exercise intolerance with symptoms such as shortness of breath and fatigue during exercise after having COVID-19.  All participants in RECOVER-ENERGIZE trials will be screened for PEM. Participants who are identified as having PEM, via a validated PEM questionnaire, will not be included in this trial. Participants will be randomly assigned to receive either personalized cardiopulmonary rehabilitation or basic exercise education for three months.
• A trial to test a program known as structured pacing, which is designed to help participants with PEM identify, control, and minimize symptoms that developed after having COVID-19 by regulating or pacing their daily activities. Currently, structured pacing is the only intervention used to treat PEM. The trial will not include any exercise training or physical movement to protect participants from developing worsened symptoms of PEM. Participants will be randomly assigned to receive either structured pacing with a trained coach or basic PEM education for three months.

All four trials were developed using comprehensive feedback from the community and in close partnership with patient representatives, whose insights were especially important for the PEM trial. The PEM trial was developed to address concerns expressed by patient advocacy groups about patient safety, and to better understand how this study program may help improve PEM symptoms.

“Structured pacing is currently the only intervention used to prevent post-exertional malaise, so we hope to test its effectiveness and determine how to best guide patients regarding activity management,” said Lucinda Bateman, M.D., an expert in PEM and founder of the Bateman Horne Center, Salt Lake City, a facility specializing in treating people with ME/CFS, long COVID and fibromyalgia.

Diversity among trial participants is a high priority for the RECOVER Initiative. To support diverse and inclusive representation, study sites are chosen based on geographic location, their connection to communities, and track record for enrolling diverse research participants. Teams at the selected study sites will recruit participants from their health systems and surrounding communities.

Sites currently activated for each trial can be found on ClinicalTrials.gov (RECOVER-SLEEP  NCT06404086 ,  NCT06404099 ,  NCT06404112 and RECOVER-ENERGIZE NCT06404047 ,  NCT06404060 ,  NCT06404073 ). New sites will be added to clinicaltrials.gov as they begin enrolling participants.

With the launch of these four studies, RECOVER is currently testing 13 treatments across eight clinical trials and continues to enroll participants. Those interested in learning more about RECOVER clinical trials should visit trials.recovercovid.org .

About RECOVER : The National Institutes of Health Researching COVID to Enhance Recovery (NIH RECOVER) Initiative brings together clinicians, scientists, caregivers, patients, and community members to understand, diagnose, and treat long COVID. RECOVER has created one of the largest and most diverse groups of long COVID study participants in the world. In addition, RECOVER clinical trials are testing potential interventions across five symptom focus areas. For more information, please visit recovercovid.org . 

HHS Long COVID Coordination: This work is a part of the National Research Action Plan , a broader government-wide effort in response to the Presidential Memorandum directing the Secretary for the Department of Health and Human Services to mount a full and effective response to long COVID. Led by Assistant Secretary for Health Admiral Rachel Levine, the Plan and its companion Services and Supports for Longer-term Impacts of COVID-19 report lay the groundwork to advance progress in the prevention, diagnosis, treatment, and provision of services for individuals experiencing long COVID.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov .

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Long Covid at Work: A Manager’s Guide

• Katie Bach,
• Ludmila N. Praslova,
• Beth Pollack

Nearly 18 million U.S. adults have long Covid, a multisystem illness that sometimes appears after a bout of Covid-19. Its wide range of symptoms vary from person to person, veer from mild to severe, and can wax and wane over time. There are no official treatments for long Covid; while some people see their symptoms resolve, others remain chronically ill. For those employees, the right workplace support can be transformative. Employers must not only help these individual employees but also build disability inclusion into their cultures and talent practices. A menu of accommodations along with individual job redesign efforts will help companies retain employees with long Covid and other chronic illnesses and enable them to contribute more than they could otherwise.

It’s time for organizations to be inclusive of employees with chronic illnesses. Here’s how.

Before the pandemic, Dara was a research engineer, thriving in a job that involved complex technical design and problem-solving. (Names in this article have been changed for privacy.) She was also an avid baker and a voracious reader. Then in March 2020, she got Covid-19. Even after the acute illness had passed, many symptoms remained: Dara struggled to sit up for more than half an hour, was too breathless and lightheaded to walk even short distances, and had severe brain fog that left her unable to hold a conversation or write an email. She used all of her paid and unpaid leave to rest and try to recover. Eventually she improved enough to return to work — but she knew her job needed to change.

• Katie Bach works with companies to improve job quality and employee experience. She has written extensively about the labor market impact of long Covid, including as a former nonresident senior fellow at the Brookings Institution, and serves as board chair of PolyBio Research Foundation , which focuses on complex chronic conditions. Follow her on LinkedIn . kathrynsbach
• Ludmila N. Praslova , PhD, SHRM-SCP, uses her extensive experience with neurodiversity and global and cultural inclusion to help create talent-rich workplaces. The author of The Canary Code , she is a professor of graduate industrial-organizational psychology and the accreditation liaison officer at Vanguard University of Southern California. Follow Ludmila on LinkedIn .
• Beth Pollack is a research scientist at MIT . She studies long Covid and associated illnesses and leads research on their overlaps and shared pathophysiology in MIT’s Tal Research Group. Beth is the chair of the ME/CFS Less Studied Pathologies Subgroup and a member of the ME/CFS Research Roadmap Working Group at the National Institutes of Health, working to create a national plan to advance research on the illness toward clinical trials. Currently collaborating on three clinical studies on long Covid and associated chronic illnesses, she is a member of the Patient-Led Research Collaborative and a former senior researcher at Harvard University. Follow Beth on LinkedIn .

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U.S. Tightens Rules on Risky Virus Research

A long-awaited new policy broadens the type of regulated viruses, bacteria, fungi and toxins, including those that could threaten crops and livestock.

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By Carl Zimmer and Benjamin Mueller

The White House has unveiled tighter rules for research on potentially dangerous microbes and toxins, in an effort to stave off laboratory accidents that could unleash a pandemic.

The new policy, published Monday evening, arrives after years of deliberations by an expert panel and a charged public debate over whether Covid arose from an animal market or a laboratory in China.

A number of researchers worried that the government had been too lax about lab safety in the past, with some even calling for the creation of an independent agency to make decisions about risky experiments that could allow viruses, bacteria or fungi to spread quickly between people or become more deadly. But others warned against creating restrictive rules that would stifle valuable research without making people safer.

The debate grew sharper during the pandemic, as politicians raised questions about the origin of Covid. Those who suggested it came from a lab raised concerns about studies that tweaked pathogens to make them more dangerous — sometimes known as “gain of function” research.

The new policy, which applies to research funded by the federal government, strengthens the government’s oversight by replacing a short list of dangerous pathogens with broad categories into which more pathogens might fall. The policy pays attention not only to human pathogens, but also those that could threaten crops and livestock. And it provides more details about the kinds of experiments that would draw the attention of government regulators.

The rules will take effect in a year, giving government agencies and departments time to update their guidance to meet the new requirements.

“It’s a big and important step forward,” said Dr. Tom Inglesby, the director of the Johns Hopkins Center for Health Security and a longtime proponent of stricter safety regulations. “I think this policy is what any reasonable member of the public would expect is in place in terms of oversight of the world’s most transmissible and lethal organisms.”

Still, the policy does not embrace the most aggressive proposals made by lab safety proponents, such as creating an independent regulatory agency. It also makes exemptions for certain types of research, including disease surveillance and vaccine development. And some parts of the policy are recommendations rather than government-enforced requirements.

“It’s a moderate shift in policy, with a number of more significant signals about how the White House expects the issue to be treated moving forward,” said Nicholas Evans, an ethicist at University of Massachusetts Lowell.

Experts have been waiting for the policy for more than a year. Still, some said they were surprised that it came out at such a politically fraught moment . “I wasn’t expecting anything, especially in an election year,” Dr. Evans said. “I’m pleasantly surprised.”

Under the new policy, scientists who want to carry out experiments will need to run their proposals past their universities or research institutions, which will to determine if the work poses a risk. Potentially dangerous proposals will then be reviewed by government agencies. The most scrutiny will go to experiments that could result in the most dangerous outcomes, such as those tweaking pathogens that could start a pandemic.

In a guidance document , the White House provided examples of research that would be expected to come under such scrutiny. In one case, they envisioned scientists trying to understand the evolutionary steps a pathogen needed to transmit more easily between humans. The researchers might try to produce a transmissible strain to study, for example, by repeatedly infecting human cells in petri dishes, allowing the pathogens to evolve more efficient ways to enter the cells.

Scientists who do not follow the new policy could become ineligible for federal funding for their work. Their entire institution may have its support for life science research cut off as well.

One of the weaknesses of existing policies is that they only apply to funding given out by the federal government. But for years , the National Institutes of Health and other government agencies have struggled with stagnant funding, leading some researchers to turn instead to private sources. In recent years, for example, crypto titans have poured money into pandemic prevention research.

The new policy does not give the government direct regulation of privately funded research. But it does say that research institutions that receive any federal money for life-science research should apply a similar oversight to scientists doing research with support from outside the government.

“This effectively limits them, as the N.I.H. does a lot of work everywhere in the world,” Dr. Evans said.

The new policy takes into account the advances in biotechnology that could lead to new risks. When pathogens become extinct, for example, they can be resurrected by recreating their genomes. Research on extinct pathogens will draw the highest levels of scrutiny.

Dr. Evans also noted that the new rules emphasize the risk that lab research can have on plants and animals. In the 20th century, the United States and Russia both carried out extensive research on crop-destroying pathogens such as wheat-killing fungi as part of their biological weapons programs. “It’s significant as a signal the White House is sending,” Dr. Evans said.

Marc Lipsitch, an epidemiologist at Harvard and a longtime critic of the government’s policy, gave the new one a grade of A minus. “I think it’s a lot clearer and more specific in many ways than the old guidance,” he said. But he was disappointed that the government will not provide detailed information to the public about the risky research it evaluates. “The transparency is far from transparent,” he said.

Scientists who have warned of the dangers of impeding useful virus research were also largely optimistic about the new rules.

Gigi Gronvall, a biosafety specialist at the Johns Hopkins Bloomberg School of Public Health, said the policy’s success would depend on how federal health officials interpreted it, but applauded the way it recognized the value of research needed during a crisis, such as the current bird flu outbreak .

“I was cautiously optimistic in reading through it,” she said of the policy. “It seems like the orientation is for it to be thoughtfully implemented so it doesn’t have a chilling effect on needed research.”

Anice Lowen, an influenza virologist at Emory University, said the expanded scope of the new policy was “reasonable.” She said, for instance, that the decision not to create an entirely new review body helped to alleviate concerns about how unwieldy the process might become.

Still, she said, ambiguities in the instructions for assessing risks in certain experiments made it difficult to know how different university and health officials would police them.

“I think there will be more reviews carried out, and more research will be slowed down because of it,” she said.

Carl Zimmer covers news about science for The Times and writes the Origins column . More about Carl Zimmer

Benjamin Mueller reports on health and medicine. He was previously a U.K. correspondent in London and a police reporter in New York. More about Benjamin Mueller