A systematic review and meta-analysis of 38 studies and 207,587 patients found that low 25(OH)D levels were associated with increased Covid-19 risk, severity, and mortality, and that vitamin D supplementation was associated with a lower risk of both Covid-19 severe disease and mortality.

Serum 25(OH)D Levels and Covid-19 Risk, Severity, and Mortality
Eight studies investigated the association between 25(OH)D levels and risk of Covid-19. All studies showed an increased risk of Covid-19 in those with low 25(OH)D levels (summary relative risk (SRR) = 2.18 (95% CI: 1.55-3.06)).

Sixteen studies investigated the association between 25(OH)D levels and COVID-19 severity and found a significant increased risk of severe disease for those with low serum 25(OH)D levels (SRR = 2.38, 95% CI: 1.53-3.70).

Nineteen studies investigated the association between 25(OH)D levels and Covid-19 mortality and found a significant increased risk of death for those with low 25(OH)D levels (SRR = 2.35, 95% CI: 1.46-3.80).

Vitamin D Supplementation and COVID-19 Severity and Mortality
Six studies investigated the association between vitamin D supplementation and Covid-19 severity and found a significant 62% reduction of severe disease with vitamin D supplementation (SRR = 0.38, 95% CI: 0.20-0.72).

Seven studies investigated the association between vitamin D supplementation and Covid-19 mortality and found a significant 65% reduction in the risk of death for subjects supplemented with vitamin D (SRR = 0.35, 95% CI: 0.17–0.70).