Long COVID immune system trapped in prolonged viral battle, UCSF study suggests

Nearly two years after their initial infection, some people with long COVID still harbor traces of the coronavirus in their intestines. A team of researchers at the University of California, San Francisco discovered this by examining tissue samples from 44 patients dealing with lingering symptoms and comparing them to 13 people who had fully recovered. What they found suggests the body's immune system is caught in a kind of stalemate—mounting an inflammatory response that fails to finish the job.

The researchers, led by Michael Peluso and Timothy Henrich, detected viral genetic material in the intestinal tissue of 27 percent of the long COVID patients, versus just one of the recovered participants. The presence of both single- and double-stranded viral RNA in the cells lining the intestine is particularly telling. These cells normally shed and regenerate within days, so finding viral material there suggests either the virus is still replicating or the tissue is being repeatedly exposed to it from somewhere else in the body. The team was careful to note that detecting genetic fragments doesn't prove infectious virus is still alive and circulating, but the pattern is suggestive.

What makes the discovery more striking is what the immune system was doing—or failing to do—in response. The researchers found extensive abnormalities in how immune cells were behaving in the intestinal tissue. Some of the body's first-line defenses remained activated months or years after the initial infection, as if still sounding an alarm. At the same time, the mechanisms needed to recognize and destroy cells that harbor the virus were weakened or impaired. It's an uncoordinated response: inflammation persisting while the tools needed to eliminate its source are dulled.

This pattern showed up much more clearly in intestinal tissue than in blood samples, which carries an important implication. Standard blood tests, the kind most doctors order, may be missing the biological changes that are actually driving long COVID. The disease appears to be playing out in the tissues, not in the circulation, which means conventional screening could give a false sense of normalcy even as something significant is happening inside the gut.

The findings, released as a preprint ahead of peer review, arrive at a moment when long COVID remains one of the pandemic's most stubborn mysteries. Millions of people worldwide are living with it—persistent fatigue, difficulty concentrating, a strange worsening of symptoms after physical exertion. Some have been dealing with it for years. Understanding what causes it has become urgent not just for those affected but for medicine more broadly, since long COVID appears to be part of a larger category of chronic illnesses that can follow infections. Insights here could reshape how doctors think about post-viral conditions generally.

The research also has immediate implications for treatment. Antiviral drugs alone haven't shown clear benefits in trials of long COVID, and these findings suggest why. If the problem is both a lingering viral presence and a compromised immune response, then targeting the virus alone won't be enough. Treatment may need to work on two fronts: clearing or controlling whatever virus remains while also restoring the immune system's capacity to eliminate infected cells. That's a more complex challenge than simply killing a virus, but it's the one the data now seems to point toward.