Vitamin D is being shown to mitigate the severity of Covid-19 infection
Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths William B. Grant, Henry Lahore, Sharon L. McDonnell, Carole A. Baggerly, Christine B. French, Jennifer L. Aliano, and Harjit P. Bhattoa Nutrients 2020, 12(4), 988 https://doi.org/10.3390/nu12040988
This recently published paper details the current research in support of higher vitamin D serum levels and decreased incidence of viral-induced respiratory diseases.
Through several mechanisms, vitamin D can reduce risk of infections.Those mechanisms include inducing cathelicidins and defensins that can lower viral replication rates and reducing concentrations of pro-inflammatory cytokines that produce the inflammation that injures the lining of the lungs, leading to pneumonia, as well as increasing concentrations of anti-inflammatory cytokines.
First data to be published on COVID-19 severity and vitamin D levels: Vitamin D Supplementation Could Possibly Improve Clinical Outcomes of Patients Infected with Coronavirus-2019 (Covid-2019) https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3571484