Six Years In: What Causes Long COVID?

For the first few years after the pandemic began, Long COVID was described primarily by symptoms of fatigue, brain fog, headaches, dysautonomia, pain, and anxiety. But over the past few years, the conversation has shifted. It is no longer a matter of if Long COVID is real, but what is causing Long COVID to persist.

What causes Long COVID?

Recent research has identified reproducible patterns involving immune dysregulation, endothelial injury, hypercoagulation, mitochondrial impairment, and reactivation of latent infections.¹ Long COVID is not a mystery syndrome. It is a multi-system inflammatory and immune disorder with measurable physiologic abnormalities.

Long COVID Is Heterogeneous

One of the most important developments over the past year is the recognition that Long COVID is not one disease. It is a collection of overlapping biological phenotypes.

Large cohort analyses, including data from the NIH RECOVER Initiative, demonstrate distinct clinical clusters.² These include:

A ME/CFS-like phenotype with post-exertional malaise and impaired bioenergetics
A dysautonomia/POTS-dominant presentation
A mast cell–driven inflammatory phenotype
Persistent cardiopulmonary injury

This heterogeneity explains why a single treatment approach fails. It also reinforces the need for individualized assessment rather than a one-size-fits-all approach.

Viral Persistence in Long COVID

Multiple studies have confirmed that SARS-CoV-2 RNA and the spike protein can persist in tissues for months and, in some cases, years after acute infection.³

Investigators have identified viral fragments in intestinal biopsies, brain tissue, and circulating monocytes. Persistent viral antigen may not represent fully replicating virus, but it appears sufficient to maintain chronic immune activation.⁴ In other words, the immune system may still be reacting to something that never fully cleared.

Immune Dysregulation, Autoimmunity, and Mast Cell Activation Syndrome

One of the most replicated findings in research studies has been altered immune signaling. Studies show:

Elevated IL-6 and TNF-α
Persistent interferon signatures
T-cell exhaustion
Altered CD4/CD8 ratios
Increased autoantibodies, including anti-nuclear antibodies (ANA) and anti-phospholipid antibodies⁵

There is also growing evidence of microglial activation in patients with neurocognitive symptoms, suggesting ongoing neuroinflammation.⁶ This immune pattern resembles similar findings in other post-infectious syndromes and autoimmune conditions.

Mast Cell Activation

Emerging data suggest that mast cell activation contributes to immune dysregulation in Long COVID. In one study, post-COVID patients demonstrated mast cell symptom burden and severity nearly identical to untreated MCAS patients, while their pre-COVID profiles resembled healthy controls.⁷

Mast cells regulate vascular tone, autonomic signaling, and inflammatory amplification. SARS-CoV-2 may trigger persistent activation through cytokine priming, Toll-like receptor signaling, autoantibody formation, and stress-induced epigenetic shifts. This model helps explain the multisystem pattern of fatigue, tachycardia, cognitive dysfunction, flushing, and GI disturbance seen in Long COVID.

Hypercoagulation and Endothelial Dysfunction

The vascular system is critical to our understanding of Long COVID. Independent research groups have documented persistent abnormalities in coagulation pathways, including platelet activation, elevated fibrinogen activity, increased thrombin generation, and impaired fibrinolysis.⁸ The coagulation is not limited to microclots.

Markers frequently observed include:

Neutrophil extracellular traps (NET) (also seen in Babesiosis)
Increased D-dimer in subsets
Increased fibrin production

The thin layer lining blood vessels, the endothelium, appears to remain inflamed long after acute infection resolves.⁹

When the coagulation system is persistently activated, blood flow efficiency decreases. Oxygen and nutrient delivery are reduced at the cellular level. Clinically, this can manifest as:

Brain fog
Fatigue
Post-exertional crashes

Mitochondrial Dysfunction in Long COVID

If inflammation persists and coagulation pathways remain activated, mitochondria suffer. Metabolic studies show impaired oxidative phosphorylation in Long COVID patients. ¹⁰ Researchers have documented:

Reduced ATP production
Elevated oxidative stress markers
Abnormal lactate responses with exertion
Impaired fatty acid oxidation

Inflammatory cytokines directly disrupt mitochondrial respiration and increase reactive oxygen species.¹¹ The result is bioenergetic failure.

This overlap with ME/CFS research has become increasingly apparent, reinforcing the mechanism of post-exertional malaise. When people say they have a limited amount of battery before becoming tired, that description is physiologically accurate.

Autonomic Nervous System Disruption

Dysautonomia, particularly POTS, has emerged as one of the most common Long COVID presentations.¹² Studies demonstrate:

Virus- or immune-mediated autonomic nervous system disruption – SARS-CoV-2 infection may directly or indirectly impair autonomic regulation.
Autoimmune mechanisms – Development of autoantibodies similar to those seen in POTS and orthostatic hypotension.
Cytokine-driven sympathetic overactivation – COVID-related inflammatory response may amplify sympathetic signaling, contributing to tachycardia and orthostatic symptoms.
Immune-mediated neuropathy – Post-viral neurological syndromes may affect autonomic fibers.

Autonomic dysfunction likely reflects immune-mediated injury combined with vascular instability.

The Gut–Immune Axis

Another area of rapid advancement has been the microbiome. Long COVID patients consistently show:¹³

Persistent gut dysbiosis with reduced microbial diversity lasting months after infection
Depletion of butyrate-producing bacteria leading to reduced SCFA production and impaired immune regulation
SARS-CoV-2–mediated ACE2 downregulation disrupting renin-angiotensin system (RAS) balance and gut microbial homeostasis
Increased intestinal permeability allowing endotoxin (LPS) translocation and systemic cytokine activation
Gut–organ axis disruption contributing to respiratory, neurologic, cardiovascular, renal, and musculoskeletal symptoms

Some studies have identified persistent viral RNA in gut tissue, suggesting the gastrointestinal tract may serve as a viral reservoir.¹⁴ The gut is one of the immune system’s largest regulatory organs. When microbial balance shifts, systemic inflammation follows.

Reactivation of Epstein–Barr Virus

An infection with the COVID virus has been shown to reactivate Epstein barr virus, possibly contributing to chronic symptoms. Below are the mechanisms that explain how this occurs.¹⁵

Acute inflammatory stress response: SARS-CoV-2 infection creates a systemic inflammatory state that acts as a physiological stressor capable of triggering latent EBV reactivation.
Cytokine-mediated immune dysregulation: Elevated inflammatory cytokines (e.g., IL-6) during COVID-19 are associated with EBV and may impair immune control of latent virus.
Lymphocyte dysfunction and immune suppression: COVID-19–associated alterations in lymphocyte subpopulations reduce immune surveillance of latently infected B cells, permitting viral reactivation.
Early reactivation during acute infection: EBV reactivation has been observed within weeks of SARS-CoV-2 infection, suggesting that immune perturbation occurs rapidly.
Latent virus vulnerability: Because >90% of adults harbor latent EBV, widespread baseline infection creates a large population of people susceptible to reactivation during immune disruption.

COVID–induced inflammation and immune dysregulation appear to destabilize immune control over latent EBV, allowing reactivation that may contribute to Long COVID symptomatology.

Reactivation of Latent Infections: Lyme and Bartonella

Most Long COVID patients I have evaluated had underlying conditions that were previously undiagnosed, including chronic infections and increased toxin burden.

A case report suggests that COVID infection may suppress the immune system, allowing Lyme disease to reactivate.¹⁶ The case described symptoms worsening following COVID, associating post-viral immune disruption as a trigger. Similarly, a 2024 report documented reactivation of Bartonella after acute COVID infection, with rapid progression of a previously regressing inflammatory mass and new pulmonary involvement occurring in parallel with SARS-CoV-2 symptoms.¹⁷ The authors proposed that immune dysregulation during COVID contributed to loss of pathogen containment.

Mechanistically, this makes sense. COVID induces lymphocyte dysfunction, cytokine shifts, endothelial activation, and systemic inflammatory stress. Intracellular infections such as Borrelia (the bacteria that cause Lyme disease) and Bartonella rely on immune surveillance for control. When that surveillance is disrupted, previously dormant organisms may proliferate or amplify inflammatory signaling. When I see this in my practice, it presents as COVID destabilizing the immune system, causing a “second hit” that unmasks deeper layers of chronic infections.

A Systems-Based Approach to Recovery

The most important lesson from the past several years of research is that Long COVID is not one. It rarely exists in isolation.

In my practice, I approach Long COVID the same way I approach other complex chronic conditions, such as Lyme disease, mold-related illness, autoimmune disorders, and chronic fatigue syndrome (ME/CFS). There are various layers contributing to a multi-system illness. The primary clinical goal is to determine what underlying causes were present before COVID and what systems became dysregulated after COVID.

When we identify the dominant causes, treatment becomes targeted. And when treatment is targeted, progress becomes possible. Long COVID is complex, but complexity calls for comprehensive, systems-based evaluation and individualized treatment, just as we would apply to any other complex chronic illness.