Staying home or in a hospital bed without sufficient sunshine exposure (which will likely happen to anyone feeling sick) will reduce vitamin D levels.
I don’t know what you consider “known way”, but this Hypothesis is mentioned in almost every discussion, and is not controlled or corrected for in any study I’ve seen.
(There is reason to believe vitamin D Supplementation is helpful, based on earlier studies related to viral respiratory diseases - but not yet enough data specifically for covid)
