Even if you're taking vitamin D supplements, you probably won't get the full anti-covid benefits unless you also get at least 15 (and preferably 30) minutes of direct sunlight exposure each day. My understanding is that vitamin D causes your skin to produce nitric oxide, but this nitric oxide doesn't actually get released into the rest of your body unless you get actual sun exposure.
> get at least 15 (and preferably 30) minutes of direct sunlight exposure each day
This time is directly proportional to how much skin you expose to the sun. If it's only face & hands you need a lot more than if you're in the swimsuit.
It would be interesting to know what clothing the 15-30 min tip assumes.
Also, sunlight through a window does nothing. You need the UV light.
Not a doctor or any profession of relevance but I do use tanning lights. Incandescents produce mostly visible light and very little UVA or UVB for those health concerns. Fluorescents tend to produce more UVB in comparison but still very negligible. Tanning lights generally produce the amount of UVA and UVB needed for vitamin D synthesis although you have to use with caution.
That isn't a source for vitamin D supplements not having an effect on COVID, that's a source saying that sunlight is needed to produce nitric oxide.
Looking online, I see that there is a hypothesis that nitric oxide has a protective effect against COVID, but I don't see that this hypothesis is any better tested than the hypothesis that vitamin D itself has a protective effect against COVID.
So, sunlight will probably get you the best of both worlds, as it would help either way, but there doesn't seem to be real evidence for nitric oxide vs vitamin D.
Yeah I would describe the evidence for both as circumstantial but fairly compelling, and very low risk. Even given the unknowns and potential risks, getting enough sun exposure is probably the best anti-Covid intervention from a cost-benefit perspective.