When I see these studies, I always picture the poor person stuck inside because they are sick, of course they have a lower vitamin D level.
I would like to see a double blind study where the sick group getting vitamin D get better.
(Sorry, working, I swear I typed all the words the first time)
This is pure speculation but...
I suspect that vitamin D is most valuable as a preventive agent. That is, to keep the body running properly so to not develop debilitating conditions. OTOH, maybe some conditions can be reversed with vitamin D supplements.