Actual experiments for causation take more time. The abstract itself addresses this: "Given the dynamics of the COVID-19 pandemic and the proven safety of vitam

Actual experiments for causation take more time. The abstract itself addresses this: "Given the dynamics of the COVID-19 pandemic and the proven safety of vitamin D supplementation, it therefore appears highly debatable and potentially even unethical to await results of such trials before public health action is taken." I guess the real question is whether you'll wish you had been taking vitamin D if you get Covid-19. While this could end up being placebo, there's some compelling evidence that you have everything to gain and nothing to lose.