A retrospective British study of approximately 1000 hospitalized COVID-19 patients found an 80% reduction in mortality with high-dose vitamin D.

(vitamin D) Ling, S.F., Broad, E., Murphy, R. et al. High-Dose Cholecalciferol Booster Therapy is Associated with a Reduced Risk of Mortality in Patients with COVID-19: A Cross-Sectional Multi-Centre Observational Study. Nutrients. 11 December 2020. https://doi.org/10.3390/nu12123799.