Two large clinical trials showed Vitamin D supplements do not reduce the risk of Covid-19.

Study 1: Effect of a test-and-treat approach to vitamin D supplementation on risk of all cause acute respiratory tract infection and covid-19: phase 3 randomised controlled trial (CORONAVIT) – The BMJ

Study 2: Prevention of covid-19 and other acute respiratory infections with cod liver oil supplementation, a low dose vitamin D supplement: quadruple blinded, randomised placebo controlled trial – The BMJ

Editorial: Can vitamin D protect against covid-19? – The BMJ

News Release: No link between vitamin D supplements and reduced risk of covid-19 – BMJ Newsroom