An interesting tweet from Dr. Tom Friedem former Director of the CDC.
Dr. Tom Frieden
DrTomFrieden
"40% or more of US adults may have a Vitamin D deficiency. Supplementation may offer vulnerable people some protection from #COVID19. Vit D appears to help protect against respiratory infections including w/ viruses such as influenza".
Also a link to the BMJ on Vitamin D and respiratory infections.