This is my understanding as a layperson who has looked into this a bit: If you look into the bradykinin hypothesis of action for Covid-19, you'll find that vitamin D and its metabolites are directly implicated in the network of effects Covid-19 has on your cellular functions. In particular, Covid-19 seems to severely down-regulate expression of certain genes, so that you would seem to be more vitamin D deficient than you would be otherwise!

https://youtu.be/DZuCrwYa80s?t=169

That's from several months ago, so maybe this isn't current research anymore.