“…In May, the U.S. Centers for Disease Control and Prevention reported that a review of the medical records of nearly 2 million people suggested at least one in five of those diagnosed with COVID-19 had developed conditions characteristic of Long Covid. Other studies have found roughly similar rates. Some recent research suggests the risk for vaccinated people is somewhat lower, but vaccination’s power to head off the syndrome remains uncertain. …”

Couzin-Frankel J.: Clues to long COVID,  Science, 376 (6599), • DOI: 10.1126/science.add429

https://www.science.org/content/article/what-causes-long-covid-three-leading-theories