This article (this article is a preprint and has not been peer reviewed yet) shows a systematic literature search that was performed in order to identify retrospective cohorts as well as clinical studies on COVID-19 mortality rates versus 25-hydroxy vitamin D serum levels.
A negative correlation was found between the levels of 25-hydroxy vitamin D and mortality. The theoretical point of zero risk for a linear relationship is at 50 mg/mL for 25-hydroxy vitamin D in serum.
Based on the multiple studies showing this relationship, taking vitamin D should be an essential preventive measure against mortality from COVID-19.