Dr. John Campbell, "Vitamin D and Immunity", Dr. John Campbell YouTube Channel (09 March 2020)
I have only very recently found Dr. John Campbell's videos, so I haven't had time to look at much of his output, but this video struck me as very informative and helpful vis-à-vis the current coronavirus pandemic. In the video, Dr. Campbell—a retired UK nurse with a PhD in nursing education—shares the findings of research published in the BMJ (British Medical Journal) which suggests that the regular taking of Vitamin D supplements (for most individuals) might well be beneficial for protection against infection.
Needless to say (though I'll say it anyway), I cannot vouch for the reliability of any of Dr. Campbell's statements, nor for the research which he cites, but I do recommend you watch the video and make up your own mind. The research which he discusses — "Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data", BMJ 2017;356:i6583 — can be found here: (https://www.bmj.com/content/356/bmj.i6583).
(Please only play this video if you are happy for YouTube to place cookies on your computer.)