A study from the University of Turku in Finland has upended a leading theory about what drives the debilitating symptoms of long COVID, finding that widespread brain inflammation is unlikely to be the culprit behind persistent fatigue, cognitive fog, anxiety, and depression. Instead, researchers discovered that the severity of these symptoms correlates with heightened activity in brain regions that regulate mood and emotions—a finding that could reshape how doctors approach treatment.
For years, scientists suspected that long COVID resulted from chronic inflammation in the brain following SARS-CoV-2 infection, similar to what happens in conditions like multiple sclerosis. This assumption made intuitive sense: if the virus triggered inflammation, persistent inflammation might explain persistent symptoms. But direct evidence remained elusive. Now, using advanced PET imaging designed to detect neuroinflammation, researchers including neuroimmunology professor Laura Airas have put that theory to the test.
The study examined 14 people with long COVID, 11 healthy controls, and 13 patients with multiple sclerosis—the latter serving as a known comparison group where brain inflammation is well-documented. All participants underwent both PET imaging and structural MRI scans, with blood samples analyzed for biomarkers of neuronal and glial damage. The results were striking: compared to MS patients, long COVID patients showed significantly lower inflammatory activity in the brain's white matter. When compared to healthy controls, no differences in markers of brain inflammation or neurodegeneration appeared at all.
"We did not observe evidence of widespread brain inflammation in patients with long COVID when compared to healthy controls," Airas said. The finding challenges the assumption that persistent inflammation is the primary driver of prolonged symptoms in all patients. Yet the study also revealed something more nuanced: individuals scanned within 16 months of infection showed higher white matter inflammatory activity than those further along in their disease course. This suggests that inflammation may be prominent immediately after infection and gradually diminish over time, rather than persisting unchanged.
The most compelling discovery, however, pointed not toward absence of change but toward different changes. Patients reporting higher levels of depression and anxiety, along with lower quality of life, showed increased cellular activity in the hippocampus and amygdala—brain regions crucial for memory, emotional regulation, and stress responses. These findings suggest that altered activation in emotion-regulating circuits, rather than widespread inflammation, may be linked to symptom severity in some long COVID patients.
The implications are significant. If brain inflammation is not the primary culprit in persistent long COVID, then therapies designed solely to reduce inflammation may miss the mark. Instead, Airas and her colleagues suggest that patients with prolonged symptoms might benefit more from treatments targeting stress and emotional regulation. This reframes long COVID not as a straightforward inflammatory disease but as a complex condition with multiple biological mechanisms operating at different disease stages.
The research doesn't close the door on understanding long COVID—it opens new ones. As millions of people worldwide continue to experience debilitating symptoms months or years after initial infection, understanding which biological processes actually drive those symptoms becomes essential. For patients searching for answers and doctors seeking effective treatments, this Finnish study offers a clearer path forward: one that looks beyond inflammation toward the brain's emotional and stress-response systems.