We also supplement with D3. I read a book by an Indian doctor that explains all the ways D is used by the body. Also that the FDA recommended amounts are close to the minimum needed but not the optimal amount you should have for good health. That was a few years pre Covid but I think it's helped keep us healthy. Especially mu husband who seemed to get a lot of colds.
My immune system is always on red alert. Always looking for an enemy to kill. So I haven't been prone to viral illnesses, except for the 3 years I took the free flu shot at my old job. I was so sick those 3 years. I caught every cold, the FLU! and had walking pneumonia twice. So I stopped taking the flu shot and was healthy again.
It could be genetic. My mom never took the flu shot and never got the flu. But two of her friends also didn't. Then every winter they would be all healthy and all their older church friends were constantly sick.
I'm a big believer in exposure builds immunity. As long as you don't get blasted by a heavy viral load the body learns how to fight it unless someone is severely immune compromised.
It was the extreme immune response to Covid that was killing people.
This is hilarious if you haven't seen it. Start at the 3 min mark.
https://www.bing.com/search?q=jon+s...efig=E4A7203F93B146328C25985FC52CB2F7&pc=LCTS