Anecdotally, my wife is doing her PhD in the states and lives in a share house with 5 other people. About 2 months ago I asked her to start taking 800UI of vitamin D, partly because she's a PhD student and never leaves her room and partly because of Covid. 3 weeks ago everyone in her house contracted covid except her.
I was told by my endocrinologist not to take more than 5000UI of D. Above 5000 is toxic.
5000UI is a huge amount. My wife is taking 800 3 times a week.
As I haven't been following this part link between Vitamin D and Covid: what is the mechanism of action?
It's not covid specific. When you have a healthy immune system, you tend to get sick less often. Vitamin D, possibly, helps the immune system. I take 2000 IUs a day and have for years.