[Florin_Andrei]

It is correlated, but I am not aware of even a single study that shows actual causal determination.

[nradov]

Are you claiming that you read all the studies linked above and none of them show actual causal determination? We can't really prove anything 100% in biology but the totality of evidence is very strong here.

[lambdaba]

I recall studies showing significant efficacy even in acute settings, which was slightly unexpected, at least to me.

[protein_lense]

This is not a study, but it does discuss the biochemistry of vitamin D and the role it plays in the immune system. It is left to the viewer to draw their own conclusions.

COVID-19 and Vitamin D | Association Between Vitamin D Deficiency and COVID-19

https://www.youtube.com/watch?v=cT1CaTv5-e4