Two recent studies have attempted to identify biomarkers for "long COVID." These biomarkers ar characteristics in human blood, tissues, or bodily fluids that can be measured and monitored, often providing indications of infection or disease. (For example, high blood pressure and high cholesterol are common biomarkers that physicians examine during routine medical appointments.)
A recent article by Thaiss and colleagues at the University of Pennsylvania, published in the journal Cell on Monday, addresses both real-world patients—some with long COVID and others who have fully recovered—and animal models.
Their research indicates that the release of interferons—a group of signaling proteins that cells send out as an alert system when there's a viral threat—may lead to a decrease in a key chemical messenger called serotonin. This, in turn, could result in cognitive effects such as memory issues or a feeling of "brain fog."
The research team was surprised to find that some individuals with long COVID still had viral fragments in their digestive system, even after they were no longer testing positive for the virus. These particles, identified through stool samples, may be sufficient to trigger the release of interferons.

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