Absolutely. Living in Alberta which is very far north, I've seen what can happen if you don't take proper vitamin d supplements regularly. Almost everyone I know is on a very high regular dosage of vitamin d over the winter and a lighter regimen over the summer.

With regards to covis I'd believe that vitamin d deficiency would likely lead to a pathway response that is significantly weaker. However I would not expect someone who OD's on vitamin d pills to be suddenly healthy.

More I'd expect people who don't have a decent vitamin d level to have a weaker immune system and thus naturally succumb to it more.