A large Israeli study found a strong link between vitamin D deficiency and covid-19 severity.

(vitamin D) Israel, A., Cicurel, A., Feldhamer, I., et al. The link between vitamin D deficiency and Covid-19 in a large population. Preprint from medRxiv and bioRxiv. 7 September 2020. https://doi.org/10.1101/2020.09.04.20188268.