Studies mentioned here in passing showed a correlation of vitamin D present in covid patients and better outcomes (but no proof of causation). My guess is that people who spend a lot of time in the sunshine - and that usually means they are getting exercise too - are probably healthier generally, and producing vitamin D is a product of that rather than a reason for it (sunshine exposure allows the body to manufacture vitamin D). This is at least anecdotally backed up by the fact that about 30 or so of us soccer players 50 years old and older (I'm 65) have continued to play regularly since last March. There have only two cases, and one was asymptomatic, the other mild. In our larger soccer community, which has continued to play regularly for most of the pandemic, there have been no deaths and only one case requiring hospitalization.