For years, the leading hypothesis for why long COVID produces brain fog, cognitive impairment, fatigue, anxiety, and depression has been persistent neuroinflammation — the idea that SARS-CoV-2 triggers ongoing inflammatory activity in the brain that continues damaging neural tissue long after the acute infection has cleared. New brain imaging data from Finland directly challenge that explanation, and in doing so, demand that researchers reconsider what is actually driving one of the most debilitating features of long COVID.
A study from the University of Turku, Finland, published in May 2026 and covered by ScienceDaily on May 28, 2026, found no evidence of widespread brain inflammation in long COVID patients when compared to healthy controls using advanced positron emission tomography (PET) and magnetic resonance imaging (MRI). Instead, the patients with the most severe long COVID symptoms showed increased activity in brain regions involved in mood, emotion, memory, and stress processing — a pattern that points toward different neurological mechanisms than inflammation.
"We did not observe evidence of widespread brain inflammation in patients with long COVID when compared to healthy controls," said Professor of Neuroimmunology and InFLAMES Research Flagship group leader Laura Airas, who led the study.
What the Imaging Study Found
The University of Turku study included three participant groups: 14 individuals with long COVID who continued to experience persistent symptoms after infection, 11 healthy control volunteers with no COVID history, and 13 patients with multiple sclerosis (MS) — a neurological disease definitively known to involve widespread brain inflammation. This three-group design was methodologically deliberate: by including MS patients as a positive control, the researchers could verify that their imaging techniques were capable of detecting real neuroinflammation while simultaneously comparing long COVID patients to both healthy individuals and a confirmed inflammatory disease group.
All participants underwent TSPO-PET imaging — a specialized PET technique using a tracer that binds to the translocator protein expressed on activated microglia and astrocytes during neuroinflammation. This is currently one of the most sensitive tools available for detecting active brain inflammation in living humans. Participants also received MRI scans to assess brain structure and white matter changes, and blood samples were analyzed for biomarkers reflecting neuronal and glial damage.
As reported by SciTechDaily, the results were clear in their direction:
- Long COVID patients vs. healthy controls : No differences in markers of brain inflammation or neurodegeneration
- Long COVID patients vs. MS patients : Significantly lower inflammatory activity in brain white matter in the long COVID group
- Patients with most severe long COVID symptoms : Increased glial activation specifically in the thalamus, amygdala, and hippocampus — regions involved in emotional processing, stress regulation, and memory formation
- Blood biomarkers : No significant elevations in neurodegeneration or neuroinflammation markers compared to controls
| University of Turku Long COVID Imaging Study | Data |
| Published / ScienceDaily coverage | May 2026 / May 28, 2026 (retrieved June 17, 2026) |
| Lead researcher | Professor Laura Airas, University of Turku (Neuroimmunology) |
| Groups studied | 14 long COVID patients; 11 healthy controls; 13 MS patients |
| Primary imaging technique | TSPO-PET (neuroinflammation detection) + MRI |
| Finding — long COVID vs. healthy controls | No difference in brain inflammation or neurodegeneration markers |
| Finding — long COVID vs. MS patients | Significantly LOWER inflammatory activity in long COVID white matter |
| Finding — most severe long COVID symptom patients | Increased activity in thalamus, amygdala, hippocampus (emotion/memory regions) |
| Blood biomarkers | No neuroinflammation or neurodegeneration elevation vs. controls |
| Dominant prior hypothesis challenged | Widespread persistent neuroinflammation as cause of brain fog |
| Emerging alternative mechanisms | Vascular dysfunction (BBB disruption); functional network disconnectivity; autonomic dysregulation |
What This Means — and What May Actually Be Driving Brain Fog
The absence of widespread neuroinflammation in long COVID patients does not mean the symptoms are not real, or that nothing is happening in the brains of people with long COVID. It means that the mechanism is different from what was hypothesized — and that different mechanisms require different research approaches and therapeutic targets.
The University of Turku finding is corroborated by a separate line of evidence from blood biomarker research. A study published in Scientific Reports (DOI: 10.1038/s41598-026-40142-0) found that circulating markers of neuroinflammation and neuronal damage were not elevated in long COVID patients compared to controls — despite persistent cognitive symptoms. The authors suggested that brain fog in long COVID may be "functional rather than structural," potentially reflecting disruption in the coordination between brain regions rather than active tissue damage.
What, then, is causing the cognitive impairment, fatigue, and neuropsychiatric symptoms? The current evidence increasingly points toward two interrelated mechanisms:
Blood-brain barrier (BBB) disruption and vascular dysfunction. A landmark 2024 study published in Nature Neuroscience by researchers at Trinity College Dublin used dynamic contrast-enhanced MRI to demonstrate BBB leakage in long COVID patients with brain fog. When the BBB — the protective barrier that prevents pathogens and inflammatory molecules from entering the brain — is compromised, inflammatory signals from the blood can reach brain tissue without the brain itself being inflamed in the classical neuroinflammation sense. This is an important distinction: BBB dysfunction can produce neurological symptoms through multiple downstream pathways, including coagulation system disruption, oxidative stress, and dysregulation of neurotransmitter-modulating systems, without triggering the microglial activation patterns that TSPO-PET imaging detects.
Autonomic nervous system dysregulation. Many of the regions showing increased activity in the most symptomatic long COVID patients in the Turku study — particularly the thalamus and amygdala — are core components of the autonomic stress-response system. Chronic activation of these regions is consistent with dysautonomia (dysfunction of the autonomic nervous system), which has been well-documented in long COVID and which can produce fatigue, cognitive fog, exercise intolerance, and cardiovascular symptoms through mechanisms that are entirely independent of classical brain inflammation.
What This Means for Long COVID Patients and Their Physicians
For the estimated 65 million people worldwide living with long COVID — approximately 10–15% of whom have neurological or cognitive symptoms — the Turku findings carry several practical implications.
First, the absence of widespread neuroinflammation on imaging does not mean symptoms are psychological in origin. The study is explicit: increased activity in emotion-regulation and stress-response brain regions in the most severely affected patients represents a real, measurable neurological finding — just not the one researchers expected.
Second, anti-inflammatory treatments targeting neuroinflammation may be less effective for most long COVID brain fog patients than previously hoped. This does not eliminate the role of targeted therapies addressing specific mediators, but it redirects attention toward vascular repair strategies, autonomic nervous system interventions, and functional rehabilitation approaches.
Third, for clinicians evaluating long COVID patients with persistent cognitive symptoms: the imaging findings suggest that vascular biomarkers (including assessment of BBB integrity where available) and autonomic function testing may be more informative than standard inflammatory markers. The symptom burden is real and documented; the mechanism is more complex than inflammation alone.
Frequently Asked Questions
What did the University of Turku long COVID brain study find?
Using TSPO-PET and MRI imaging in 14 long COVID patients, 11 healthy controls, and 13 MS patients, the study found no evidence of widespread brain inflammation in long COVID patients compared to healthy controls. Patients with the most severe symptoms showed increased activity in emotional processing regions (thalamus, amygdala, hippocampus) — not classical neuroinflammation.
Does this mean long COVID brain fog isn't real?
No. The symptoms are real and measurably documented. The finding means that widespread neuroinflammation is not the mechanism — not that the condition is psychological or fabricated. The increased brain activity in emotion/stress regions observed in the most severely affected patients is a real neurological finding.
If it's not inflammation, what is causing long COVID brain fog?
Current evidence points toward two main mechanisms: blood-brain barrier (BBB) disruption and vascular dysfunction (documented by Nature Neuroscience 2024 using DCE-MRI), and autonomic nervous system dysregulation. Both can produce cognitive symptoms without classical widespread neuroinflammation.
What does the blood-brain barrier have to do with brain fog?
The BBB protects the brain from inflammatory molecules in the bloodstream. When it is disrupted (as documented in long COVID brain fog patients by Trinity College Dublin researchers), inflammatory signals from systemic inflammation can affect brain function without the brain itself showing classical microglial neuroinflammation. This distinction is why TSPO-PET scans can be negative while symptoms remain severe.
What should long COVID patients with cognitive symptoms do?
Discuss your symptoms with a physician who specializes in post-COVID care. Treatments targeting BBB repair, autonomic nervous system dysfunction, and functional rehabilitation may be more relevant than anti-inflammatory approaches for most patients with neurological long COVID. The finding does not change the urgency of seeking care — it changes the direction of the relevant research and therapeutic targets.
