I have no idea about vitamin D and Covid but as someone said above most older Americans are deficient in vitamin D. Blood test indicated that my wife and I were both deficient years ago. I was able to raise my levels sufficiently by taking 1000 units twice per day. It’s cheap, easy and definitely has health benefits. If you’re over 50 have your doctor include a vitamin D test when you do blood for your next physical. No need for mega doses but if you’re low you should bring it up to “normal” levels.