Vitamin D to prevent COVID-19: recommendations for the design of clinical trials.

The coronavirus disease 2019 (COVID-19) pandemic has focused attention on the potential role of vitamin D supplementation to prevent COVID-19. In this issue, Merzon and colleagues report epidemiologic data on the vitamin D status of 7807 individuals and their risk of developing COVID-19. In multivariable analyses, low vitamin D status was associated with increased risk of both COVID-19 infection and hospitalization. The authors call for clinical trials of vitamin D supplementation. In this Commentary, we discuss some of the challenges of vitamin D research and provide recommendations for the design of randomized controlled trials of vitamin D supplementation to prevent COVID-19.