Randomized but unblinded experiment on vitamin D as a coronavirus treatment. Let’s talk about what comes next. (Hint: it involves multilevel models.) — Andrew Gelman

Of interest statistically.

Statistical Modeling, Causal Inference, and Social Science
Randomized but unblinded experiment on vitamin D as a coronavirus treatment. Let’s talk about what comes next. (Hint: it involves multilevel models.)
Andrew Gelman | Professor of Statistics and Political Science and Director of the Applied Statistics Center, Columbia University

Another study

PLOS
Vitamin D sufficiency, a serum 25-hydroxyvitamin D at least 30 ng/mL reduced risk for adverse clinical outcomes in patients with COVID-19 infection
Zhila Maghbooli,Mohammad Ali Sahraian ,Mehdi Ebrahimi,Marzieh Pazoki,Samira Kafan,Hedieh Moradi Tabriz,Azar Hadadi,Mahnaz Montazeri,Mehrad Nasiri,Arash Shirvani,Michael F. Holick