https://astralcodexten.substack.com/p/covidvitamin-d-much-more-than-you

Most health articles ask you to act on their opinions. I am specifically asking you not to act on mine. In a moment, I'll tell you whether or not I think Vitamin D prevents or treats coronavirus. But I'll give you a free spoiler: I am less than 100% certain of what I'm about to say. So if you want to take Vitamin D, take it. If it does prevent or cure coronavirus, great. If not, the worst that will happen is you'll have slightly better bone health. I can't stress how much I don't want to be those people who said they couldn't prove face masks helped so you must not use face masks. Just ignore everything I'm saying, do a quick cost-benefit calculation, and take Vitamin D. That having been said:

Lots of people think Vitamin D treats coronavirus, and some of them have good evidence. For example, infection rate from coronavirus seems latitude dependent; in general, the further north an area, the worse it's been hit. Northern areas get less sunlight, and sunlight helps produce Vitamin D, so whenever you see a disease that's worse at high latitudes, Vitamin D should be on your short list of potential causes.

Also - in the US, COVID seemed to remit with the summer and worsen over the winter. It's hard to distinguish this from general exponential growth and from the effect of playing ping-pong with gradually loosening/ tightening lockdowns, but the US spike this winter was pretty dramatic. Most Northern Hemisphere countries show such a pattern, most equatorial countries don't, and some Southern Hemisphere countries arguably show the opposite. Whenever you see a disease that's better in summer and worse in winter, Vitamin D is one of the possible culprits.