Scientists say that drugs that calm a key part of the immune system should be studied in clinical trials as potential long Covid treatments.

Scientists have identified a persistent change in a handful of blood proteins in people with long Covid that indicates that an important part of their immune system remains on high alert for months after an acute infection.

The findings, published Thursday in the journal Science, could help explain what causes the persistent fatigue, brain fog and other debilitating symptoms of long Covid, as well as pave the way for diagnostic tests and potentially, a long-awaited treatment, experts say.

The study followed 113 Covid patients for up to one year after they were first infected, along with 39 healthy controls. At the six-month mark, 40 patients had developed long Covid symptoms.

Repeated blood samples turned up important differences in their blood: A group of proteins indicated that a part of the body’s immune system called the complement system remained activated long after it should have returned to normal.