This year I started supplementing with 2000 UI of vitamin D daily and it's as if my immune system started functioning normally, in the last 10 months I only caught a really light cold that I could barely notice, whereas before this the winter season alone would bring 2 severe cold or flu episodes that would knock me down for several days.
I can't really say if it's the vitamin D, or other changes in my life, like losing some weight or walking more, or the cold exposure I've been practicing (primarily because I heard it burns calories :-)), it might be even placebo, I haven't isolated these variables well, but my wife and close ones are amazed by the transformation. E.g. 3 weeks ago everyone in my household got the flu, except for me.
So now I got everyone in my household on vitamin D, especially b/c of this coronavirus scare. During summer time we might not need it, but the sun doesn't see us during winter. And vitamins are probably no defense against the coronavirus, but a deficiency could amplify the damage (as vit D has a role in our immune system) so it's best to ensure that we aren't deficient. Also we tolerate large doses of vitamin D, so it can't hurt.