I found this article a couple of years back ... very interesting how low levels of D appear to be implicated with “regular” influenza & now also COVID.
This is an article on the Vitamin D “Hammer”. It is a serious article by doctors who have used this in their practice. It’s only 4 paragraphs ... worth your time.
Vitamin D for influenza
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4463890/
I take 5000 units of vitamin D every day because the chemo I had several years ago killed my body’s ability to make it.