There must be very well controlled groups of elderly people who after some blood test have been taking supplements and checking their vitamine D levels. I wonde

There must be very well controlled groups of elderly people who after some blood test have been taking supplements and checking their vitamine D levels. I wonder how the virus is affecting this group compared with the general population of the same age. If this group were doing well, I think it would be quite conclusive, since it would show that people under treatment (and therefore with some previous health problem) are doing better than supposedly healthy people with low vitamine D values.