Another explanation, experts say, could be that your immune system never fully shuts down after the initial infection.
Who is at risk?
Research offers some hints about which patients might face a greater risk of long-term symptoms. In a study of 209 patients published in January, researchers found four factors that could be identified early in a person’s coronavirus infection that appeared to correlate with an increased risk of having ongoing symptoms two to three months later.
One factor was the level of coronavirus RNA in the blood early in the infection, an indicator of viral load. Another was the presence of autoantibodies — antibodies that mistakenly attack tissues in the body as they do in conditions like lupus and rheumatoid arthritis. A third factor was the reactivation of Epstein-Barr virus, which can cause mononucleosis and infects most people, often when they are young, and then usually becomes dormant.
The fourth factor was having Type 2 diabetes, although experts say that in studies involving larger numbers of patients, diabetes might be only one of several medical conditions that increase the risk of long Covid.
May 20, 2022, 8:45 p.m. ET
Studies from post-Covid clinics have also found other pre-existing medical conditions that may put people at risk for long Covid. In a report on the first 100 patients treated for neurological and cognitive symptoms at a post-Covid clinic at Northwestern Memorial Hospital in Chicago, 42 percent reported previously having depression or anxiety, though such patients might simply be more comfortable seeking neurological treatment, doctors said. Other pre-existing conditions included autoimmune diseases and headaches.
Studies also suggest that the risk of developing long Covid peaks in middle age, Dr. Peluso said. The average age of patients in the Northwestern study was 43. An analysis of 78,252 private health insurance claims across the United States found that people between the ages of 36 and 64 made up about two-thirds of the long Covid patients. (But that study did not include most Medicare recipients, so it involved relatively few older patients.)
Women may be disproportionately affected, with some studies finding that about 60 percent of patients are female. A similar pattern has emerged in other long-term conditions like ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome), which has several symptoms similar to those of long Covid.