Long COVID and Inflammation: New Research Challenges Existing Theories

For many experiencing long COVID, lingering symptoms like fatigue and brain fog remain a frustrating mystery. Recent research is challenging previous assumptions about the role of persistent inflammation in the condition, suggesting the underlying mechanisms may be more complex than initially thought. A new study published in Scientific Reports indicates a lack of detectable neuroinflammation or systemic inflammation in patients nearly two years after initial SARS-CoV-2 infection.

Understanding Long COVID Prevalence and Early Hypotheses

Long COVID, characterized by symptoms persisting weeks or months after the acute phase of COVID-19, has become a significant global health concern. Estimates suggest the prevalence has risen dramatically, from approximately 60 million cases in 2020 to 400 million in 2024. ¹ Early theories proposed that long COVID might resemble other post-infectious syndromes involving ongoing organ damage or persistent viral reservoirs. Some research similarly pointed to target organ involvement, including neuronal cell damage, as a potential driver of chronic symptoms.

However, many of these early studies were conducted relatively soon after the initial infection, potentially capturing ongoing inflammation or viral activity during the healing process. As symptoms evolved, cognitive impairment and fatigue emerged as dominant features, leading researchers to explore the possibility of chronic inflammation despite normalizing biomarker levels.

Study Design and Methodology

The recent study, conducted at Stavanger University Hospital in Norway, aimed to assess circulating biomarkers of neuroinflammation and systemic inflammation in long COVID patients. ¹ Researchers conducted a case-control study involving 96 participants (48 with long COVID and 48 recovered controls) with a median of 69 weeks post-SARS-CoV-2 infection. Participants meeting the National Institute for Health and Care Excellence (NICE) criteria for long COVID – persistent symptoms for over 12 weeks unexplained by another diagnosis – were included.

To minimize confounding factors, individuals with pre-existing chronic inflammatory diseases, autoimmune conditions, cancer, or other conditions affecting fatigue were excluded. Researchers measured several key biomarkers, including:

• Neurofilament light (NfL): A biomarker of neuronal damage
• Glial fibrillary acidic protein (GFAP): A biomarker of neuroinflammation
• Triggering receptor expressed on myeloid cells 2 (TREM2): Involved in immune response
• C-reactive protein (CRP): A marker of systemic inflammation
• Tumor necrosis factor-α (TNF-α): A pro-inflammatory cytokine
• Interleukin-6 (IL-6): A pro-inflammatory cytokine

Biomarkers were measured using both ultrasensitive NULISA™ technology and standard laboratory methods.

Key Findings: No Evidence of Persistent Inflammation

The study revealed no significant differences in NfL or GFAP levels between long COVID patients and recovered controls, suggesting a lack of ongoing neuronal injury or neuroinflammation. ² Routine immunoassays also showed no significant differences in inflammatory markers between the two groups.

While unadjusted analyses using ultrasensitive assays showed nominally elevated levels of CRP, TNF-α, IL-6, and TREM2 in long COVID patients, these differences were not statistically significant after correcting for multiple comparisons. No correlation was found between inflammatory biomarker levels and symptom severity.

What Does This Imply for Long COVID Research?

These findings challenge the notion that persistent systemic inflammation, neuroinflammation, or neuronal injury are primary drivers of long COVID symptoms at this stage of the illness (69 weeks post-infection). ¹ The authors suggest that the lack of detectable inflammation may be due to the longer follow-up period, allowing sufficient time for the resolution of acute inflammation and viral clearance.

It’s important to note the study’s limitations, including its relatively small sample size, cross-sectional design, and reliance on blood-based biomarkers. The authors acknowledge that extremely low-level immune activation, potentially below current detection thresholds, or other mechanisms not assessed in this study could still contribute to symptoms.

COVID-19’s Unique Impact on the Brain

While this study doesn’t support widespread inflammation as the cause of long COVID, other research highlights the unique ways COVID-19 can affect the brain. A study from Tulane University found that, unlike the flu, COVID-19 causes small blood vessel injury and ongoing brain inflammation even after the virus is cleared. ³ This disruption specifically affects serotonin and dopamine signaling, neurotransmitters crucial for mood, cognition, and energy, potentially explaining the “brain fog” and fatigue common in long COVID. ³

Future Directions

Further research is needed to fully understand the complex mechanisms underlying long COVID. Larger studies incorporating neuroimaging, cerebrospinal fluid analysis, and a broader range of biomarkers are crucial. Investigating the role of the body’s immune response, even in the absence of detectable inflammation, remains a key priority. ⁴