Consider a recent study from the Centers for Disease Control and Prevention that claims to find that nearly 36% of Covid cases among students, faculty and staff at George Washington University resulted in "long Covid." The study suggests ... that the unvaccinated were at more than twice as high a risk of developing long Covid as those fully vaccinated who had gotten boosters. This sounds plausible. But drill down, and it becomes clear that the evidence is too thin to draw any conclusions. The study ... doesn't include a control group. The finding that nearly 36% reported long Covid symptoms is meaningless without such a sample to determine how common such symptoms were among people who never had Covid. Long Covid in general isn't well-defined, but the study defines it expansively to include problems common among college students"difficulty making decisions, fatigue, anxiety, sadness, trouble sleeping and the catch-all "other symptoms." If a student reported at least one physical or psychological problem, he was classified as having long Covid. A CDC survey in January 2021 reported that 57% of respondents between 18 and 29 had experienced anxiety or depression within the previous seven days. A November 2021 study ... found that many people with persistent physical symptoms that are commonly ascribed to long Covid didn't test positive for antibodies. A belief that one had Covid was more strongly associated with physical symptoms than a lab-confirmed infection.