Vitamin D for COVID-19: real-time meta analysis of 142 studies
https://vdmeta.com/
Unfortunate URL, but lots of good info and links to studies.
“85% of 53 vitamin D treatment studies report positive effects. 27 studies show statistically significant improvements in isolation (21 for the most serious outcome).”
The benifits of vitamin D on the immune system have been discussed since the 1960s. And the uselessness of paper masks has been talked about just as long.