Long Covid

Years after the first SARS-CoV-2 infections, Long COVID remains an important and unresolved biomedical research challenge. Rather than representing a single disease mechanism, increasing evidence suggests that persistent symptoms can arise from several overlapping biological processes involving viral persistence, immune dysregulation, autoimmunity and vascular dysfunction.

Long COVID, also referred to as post-COVID-19 condition (PCC) or post-acute sequelae of SARS-CoV-2 infection (PASC), encompasses a broad spectrum of symptoms that persist or develop after the acute phase of COVID-19. Frequently reported manifestations include fatigue, post-exertional malaise, cognitive impairment, shortness of breath and cardiovascular or neurological symptoms. The heterogeneity of these manifestations is mirrored by an increasingly complex picture of the underlying biology.

The scale of the condition continues to make understanding these mechanisms a major research priority. In its August 2026 global risk assessment, the World Health Organization estimated that approximately 6% of people with symptomatic SARS-CoV-2 infection develop post-COVID-19 condition, with around 15% of affected individuals experiencing persistent symptoms at 12 months. At the same time, substantial research programs have been established to investigate its pathophysiology. In the United States, the NIH RECOVER Initiative has received approximately $1.8 billion in initial and additional funding, while the European Commission reports investments of approximately €66 million in large cohort studies investigating the longer-term consequences of COVID-19.

Despite this effort, no single molecular explanation has emerged. Instead, current research points towards a network of potentially interconnected mechanisms. SARS-CoV-2 antigens or viral material may persist in tissues after acute infection; infection may alter control of latent viruses such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV); prolonged antigenic stimulation may contribute to sustained immune activation; and altered B-cell responses and autoantibodies could interfere with normal cellular functions. In parallel, endothelial and microvascular abnormalities may connect immune dysregulation with some of the systemic manifestations observed in affected individuals.

Understanding how these processes interact — and whether distinct mechanisms predominate in different patient subgroups — is now a central goal of Long COVID research. The following sections examine the emerging molecular evidence behind these hypotheses and highlight experimental targets and tools that can support their investigation.

SARS-CoV-2 Persistence: Viral Reservoirs After Acute Infection

One of the most intensively investigated hypotheses in Long COVID is that SARS-CoV-2, or components of the virus, can persist after the acute infection has resolved. Instead of being completely cleared, viral RNA and proteins have been detected months later in blood and in a range of tissues, including the gastrointestinal tract, lung, lymphoid tissue and other organs. Such persistence could provide a continuing source of viral antigens capable of stimulating the immune system long after respiratory symptoms and acute viral replication have subsided.

Evidence for tissue persistence has become increasingly substantial. In a 2024 cohort study of 225 individuals recovering from predominantly mild COVID-19, SARS-CoV-2 RNA was detected in multiple tissues, including the intestine, stomach, liver, kidney, lung, blood vessels, skin and thyroid. Viral RNA was still detectable in 11% of tissue samples collected approximately four months after infection. Importantly, individuals in whom viral RNA was detected were more likely to report Long COVID symptoms, and higher viral copy numbers were associated with a greater probability of persistent symptoms. Other studies have identified SARS-CoV-2 RNA together with prolonged local T-cell activation in tissues such as the gut for considerably longer periods following infection.

Persistent viral material can also be detected without direct tissue sampling. Ultrasensitive assays have identified circulating SARS-CoV-2 Spike, S1 and Nucleocapsid antigens during the post-acute phase of infection. However, the relationship between antigen persistence and Long COVID is not straightforward. A two-year longitudinal study published in 2026 detected circulating SARS-CoV-2 antigens in 31% of individuals with Long COVID at 6–12 months after infection, compared with 20% of fully recovered individuals. By 18–24 months, antigen detection had fallen markedly. Antigenemia was also not associated with the number or type of Long COVID symptoms. Persistent antigen therefore appears to characterize a subset of post-infection individuals rather than providing a universal explanation or biomarker for Long COVID.

A further distinction is important: detection of viral RNA or proteins does not necessarily demonstrate the presence of replication-competent SARS-CoV-2. Persisting RNA fragments, viral proteins, infected long-lived cells and sites of ongoing viral replication represent biologically different scenarios. Determining which of these forms of persistence occur in Long COVID — and whether they actively sustain inflammation — remains an important research question. Studies investigating this mechanism therefore combine molecular detection of viral RNA with assays targeting viral antigens such as Spike, S1 and Nucleocapsid, tissue localization techniques and measurements of the accompanying cellular and humoral immune response.

Latent Virus Reactivation After SARS-CoV-2 Infection

Most adults carry persistent viruses that are normally kept under control by immune surveillance. SARS-CoV-2 infection can disturb this balance. Reactivation has been reported particularly for members of the Herpesviridae family, including Epstein-Barr virus (EBV) and cytomegalovirus (CMV), raising the possibility that the virological consequences of COVID-19 extend beyond SARS-CoV-2 itself.

A 2026 Nature study investigating 1,154 patients hospitalized with COVID-19 detected extensive reactivation of chronic viruses, including Herpesviridae and Anelloviridae. Viral reactivation was associated with distinct changes in inflammatory cytokines, immune-cell populations and metabolism, and some viral activity persisted into convalescence.⁶ Earlier longitudinal multi-omics research had also identified EBV viremia during acute COVID-19 as one of several factors associated with the subsequent development of post-acute sequelae.⁷

Whether EBV or other latent viruses actively drive Long COVID, however, or instead reactivate as a consequence of an already dysregulated immune system remains unresolved. The available associations do not establish causality. Nevertheless, reactivation could increase the antigenic and inflammatory burden encountered by the immune system, while impaired control of latent viruses may itself indicate persistent changes in immune regulation. Both possibilities provide a direct link from viral persistence and reactivation to the prolonged immune abnormalities observed in Long COVID.

Persistent Immune Activation and Dysregulation

Persistent immune dysregulation is emerging as a potential point of convergence between several Long COVID mechanisms. Continued exposure to viral antigens, reactivation of latent viruses or tissue damage could maintain immune signaling after the acute infection, while altered immune-cell function may in turn reduce the ability to resolve these stimuli.

Deep immune profiling has identified changes in both cellular and soluble immune responses months after SARS-CoV-2 infection. Individuals with Long COVID have shown altered T-cell subset distributions, increased frequencies of CD4+ T cells with tissue-homing characteristics, exhausted SARS-CoV-2-specific CD8+ T cells and poorly coordinated virus-specific T- and B-cell responses.⁸ More recent transcriptomic and proteomic analyses found persistent activation of pro-inflammatory pathways beyond 180 days, including IL-6/JAK-STAT signaling, complement-associated pathways and signatures of T-cell exhaustion.⁹

These findings do not define a single Long COVID immune phenotype. Hyperactivation, exhaustion and altered adaptive responses can occur in different combinations, consistent with the existence of multiple biological endotypes. Persistent disturbances of B-cell activation and immune regulation are particularly relevant to the next proposed mechanism: the production and persistence of antibodies directed against self-antigens.

Autoantibodies and Autoimmune Mechanisms in Long COVID

Acute viral infections can transiently disrupt immune tolerance and stimulate antibodies that recognize host molecules. Autoantibodies have consequently been investigated in Long COVID for several years, although their presence alone does not establish that they are pathogenic. The important question is whether particular autoantibody populations directly contribute to specific Long COVID phenotypes.

Recent experimental studies have provided stronger evidence for such a causal role in subsets of patients. In 2026, researchers identified autoantibodies against multiple central and peripheral nervous system proteins in people with neurological Long COVID. Transfer of patient-derived IgG into mice induced fatigue-like behavior, impaired coordination, thermal hypersensitivity and small-fiber nerve damage.¹⁰ An independent study similarly found that IgG isolated from individuals with Long COVID induced persistent mechanical hypersensitivity in mice, including when antibodies were obtained from patients who remained symptomatic two years later.¹¹

These experiments move the evidence beyond a simple association between autoantibodies and symptoms, but they should not be interpreted as evidence that autoimmunity explains Long COVID universally. Instead, pathogenic IgG may define particular disease endotypes. Autoantibodies can also interact with Fc-receptor pathways, immune cells and the complement system, providing another potential route through which persistent immune dysregulation may translate into tissue and vascular effects.

Complement Activation, Endothelial and Microvascular Dysfunction

The complement system forms an important interface between antibodies, innate immunity, inflammation and coagulation. Persistent complement activation could therefore connect upstream processes such as viral antigens, reactivated viruses or autoreactive antibodies with downstream endothelial and microvascular dysfunction.

Longitudinal proteomic analysis has identified dysregulation of the classical, alternative and terminal complement pathways in active Long COVID, together with markers of hemolysis, tissue injury, platelet activation and monocyte-platelet aggregates.¹² Independent analyses have reported increased activation products including C1s-C1INH, Ba, iC3b, C5a and the terminal complement complex, while more recent work also implicates persistent activation of the lectin pathway.^13,14

These immune abnormalities occur alongside evidence of endothelial activation. Increased circulating levels of factors such as von Willebrand factor (VWF), platelet factor 4 (PF4), E-selectin and PECAM-1 have been reported in Long COVID cohorts.¹⁵ Complement activation, endothelial injury and platelet responses can amplify one another, potentially impairing normal microvascular function. As with the other mechanisms discussed here, however, these abnormalities are heterogeneous and it remains unclear whether vascular dysfunction represents a primary driver in particular patients or a downstream consequence of persistent immune activation.

Taken together, current evidence supports a model in which Long COVID can emerge from several interacting biological processes rather than a universal linear pathway. Viral persistence may sustain antigen exposure, SARS-CoV-2 infection may alter control of latent viruses, and prolonged immune activation may favor autoreactive responses. Antibodies, complement and inflammatory mediators can subsequently interact with vascular and tissue compartments. Determining which of these mechanisms dominate in individual patients will be central to defining biologically meaningful Long COVID subgroups and developing more targeted experimental and therapeutic approaches.

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