> enrolled 431 adults from the general population with SARS-CoV-2 infection

How do you control for people’s willingness to enter a study? If I had a mild case of Covid and no ongoing symptoms, I wouldn’t be too interested in participating in a study. But if I had serious ongoing symptoms, I’d be very interested. It’s a voluntary study, so how in the world do you control for that? If you can’t control for that, it skews the results to make long COVID seem much more common.