Long COVID has left millions with ongoing fatigue, cognitive issues, heart rate and blood pressure variation, and other debilitating symptoms. Recent findings reported in the journal Nature may help scientists develop a blood test to diagnose the illness. The study involved a cross section of 268 individuals—152 with long COVID and 116 without it as a control group. 

Blood testing revealed significant differences between the two test groups. Long COVID patients exhibited irregularities in the activity of immune system T cells and B cells that help fight off germs, as well as significantly lower levels of cortisol, a hormone that helps people feel alert and awake. Dormant viruses, such as Epstein-Barr that causes mononucleosis, were activated in the long COVID patients.

While the scientists did not find significant evidence that long COVID was the result of an autoimmune disorder in which the body attacks itself, they suggested future studies would be needed to identify immune system problems. They also recommended further inquiry to understand cortisol’s role in long COVID and to determine whether dormant viruses are causing symptoms. Long COVID affects one in 13 Americans, according to a survey  by the U.S. Centers for Disease Control and Prevention.