Nikhil Prasad Fact checked by:Thailand Medical News Team Jul 30, 2026 1 hour ago
Medical News: Researchers Find Immune Cells May Stay Overactive Long After COVID-19 Infection
Scientists have uncovered fresh evidence that may help explain why some people continue to suffer from long COVID months or even years after their initial infection. The new study found that a group of immune cells known as CD8+ T cells remain unusually aggressive in people with long COVID, continuing to target not only SARS-CoV-2 but also common herpesviruses such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV).
Researchers discover that persistent overactive killer T cells targeting multiple viruses may play a central role in long COVID
The research was conducted by scientists from the Gladstone Institutes, University of California San Francisco (UCSF), Fred Hutchinson Cancer Center, San Francisco VA Medical Center, and Zuckerberg San Francisco General Hospital, all in the United States.
Why These Immune Cells Matter
CD8+ T cells are often described as the immune system's "killer cells." Their job is to locate and destroy virus-infected cells. Normally, after an infection has been cleared, these cells gradually calm down and return to a resting state.
However, the researchers found that in people with long COVID, many of these killer T cells remain highly active months after the initial infection. Instead of shutting down, they continue showing signs of intense virus-fighting activity, which could contribute to ongoing inflammation and persistent symptoms.
Researchers Examined Multiple Viruses
Instead of looking only at SARS-CoV-2, the team also investigated immune responses against EBV and CMV, two herpesviruses that remain dormant in most people but can reactivate under certain conditions.
Using an advanced immune-cell profiling technique capable of examining dozens of viral targets simultaneously, the scientists discovered remarkably similar abnormalities across T cells recognizing all three viruses.
This
Medical News report highlights that the findings suggest long COVID is associated with a broader disruption of immune regulation rather than an isolated response to SARS-CoV-2 alone.
Key Findings Reveal Persistent Immune Dysfunction
Participants with long COVID had significantly higher numbers of terminally differentiated CD8+ T cells. These cells expressed high levels of molecules including granzyme B, CD57, and CD29, all markers of highly cytotoxic cells capable of destroying infected tissue.
At the same time, the researchers found fewer youthful, stem-like memory T cells that normally help maintain balanced immune responses and support long-term recovery.
Many of the persistent killer T cells also displayed features associated with immune exhaustion, including increased expression of PD1, TIGIT, and CD39. Although exhausted cells are often thought to function poorly, they can still remain highly inflammatory and continue damaging tissu
es. The combination of cytotoxicity and exhaustion suggests the immune system remains trapped in an unresolved battle long after acute COVID-19 has ended.
Women Showed the Strongest Changes
One of the study's most striking observations was that women with long COVID had the greatest expansion of highly cytotoxic CD8+ T cells.
Female participants showed significantly higher levels of granzyme B-positive, CD29-positive, and CD57-positive killer T cells than recovered individuals. This finding may help explain why long COVID has consistently been reported more frequently in women, although additional research will be needed to fully understand the biological reasons behind this difference.
A Possible Explanation for Long COVID
The researchers believe these persistent immune cells resemble those normally seen during severe acute COVID-19, suggesting the body's antiviral response may never completely switch off in some individuals.
Rather than fully returning to normal after infection, the immune system may remain locked in a prolonged activated state, potentially fueled by lingering viral material or repeated stimulation from dormant herpesviruses. This prolonged immune activation could contribute to fatigue, brain fog, muscle pain, and many other symptoms commonly experienced by people living with long COVID.
Conclusion
The study provides compelling evidence that long COVID is associated with persistent populations of highly cytotoxic CD8+ T cells directed against SARS-CoV-2, EBV, and CMV. These findings strengthen the theory that an unresolved immune response, rather than simply lingering infection alone, may be driving long-term illness. Understanding how these overactive immune cells develop and why they fail to return to a resting state could eventually lead to targeted treatments capable of reducing chronic inflammation and improving recovery for millions of long COVID patients worldwide.
The study findings were published in the peer reviewed journal: Cell Reports Medicine.
https://www.cell.com/cell-reports-medicine/fulltext/S2666-3791(26)00363-0
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Medical News.
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https://www.thailandmedical.news/articles/long-covid
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