Trends in Molecular Medicine Insights from Myalgic Encephalomyelitis/Chronic Fatigue Syndrome May Help Unravel the Pathogenesis of Post-Acute COVID-19 Syndrome Anthony L. Komaroff, W. Ian Lipkin Highlights • In some people, the aftermath of acute COVID-19 is a lingering illness with fatigue and cognitive defects, known as post-COVID-19 syndrome or “long COVID”. • Post-COVID-19 syndrome is similar to post-infectious fatigue syndromes triggered by other infectious agents, and to myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), a condition that patients often report is preceded by an infectious-like illness. • ME/CFS is associated with underlying abnormalities of the central and autonomic nervous system, immune dysregulation, disordered energy metabolism and redox imbalance. It is currently unclear if the same abnormalities will be identified in post-COVID-19 syndrome. • The US and other developed nations have committed considerable support for research on post-COVID illnesses. Abstract: SARS-CoV-2 can cause chronic and acute disease. Post-Acute Sequelae of SARS-CoV-2 infection (PASC) include injury to the lungs, heart, kidneys and brain, that may produce a variety of symptoms. PASC also includes a post-COVID-19 syndrome (“long COVID”) with features that can follow other acute infectious diseases as well as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Here we summarize what is known about the pathogenesis of ME/CFS and of acute COVID-19, and speculate that the pathogenesis of post-COVID-19 syndrome in some people may be similar to that of ME/CFS. We propose molecular mechanisms that might explain the fatigue and related symptoms in both illnesses, and suggest a research agenda for both ME/CFS and post-COVID-19 syndrome.