The most severe form of long COVID overlaps with myalgic encephalitis/chronic fatigue syndrome (ME/CFS), a disease that has long been considered an infection-associated chronic condition. The consensus definition of ME/CFS requires a substantial reduction or impairment in the ability to engage in pre-illness levels of activities for at least 6 months accompanied by fatigue, post-exertional malaise, unrefreshing sleep, and either cognitive impairment or orthostatic intolerance. In the national RECOVER initiative, 4.5% of study participants met criteria for ME/CFS, including 1.6% of those who enrolled in RECOVER at the time of acute COVID-19, an incidence rate of 2.66 per 100 person-years (vs 0.93 for uninfected controls).17
