Vitamin D3 should be taken daily in the winter time. Deficiency in D is probably why people get “seasonal” flu and colds. But Vitamin C, especially the more digestible Liposomal C, is perhaps the #1 thing to take daily. Humans are unique in that they don't make their own Vitamin C. It's theorized that that's why humans get clogged arteries, which no other animals get. Zinc is especially good for viruses, and in fact, the loss of smell associated with Covid is a classic symptom of zinc deficiency; it's possible the body uses up its zinc fighting Covid. The last one, 10-1 concentrate oregano oil, do not take unless you are coming down with something. It is especially useful for lung problems (like cytokine storm or pneumonia). Here's where I get mine:
I take 4 if I ever start to come down with something. A couple of weeks ago I got an ear infection so that it sounded like I had a sea shell up to my ears. That knocked it out promptly. If you get a few of these from Swanson, you can get the other stuff from them too, because you get free shipping with a $50 order.
UPDATE: Apparently, lysine is good against coronaviruses, too.