I am not seeing the theory of Vitamin D deficiency being highly correlated with severe outcomes from Covid-19 infections being suggested in the article. Please

I am not seeing the theory of Vitamin D deficiency being highly correlated with severe outcomes from Covid-19 infections being suggested in the article. Please see links below for this theory: 1) https://www.bmj.com/content/369/bmj.m1548/rr-6 https://www.bmj.com/content/369/bmj.m1548/rr-6 2) https://www.researchsquare.com/article/rs-21211/v1 https://www.researchsquare.com/article/rs-21211/v1 3) https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3585561 https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3585561