Vitamin D levels have been shown to be negatively correlated with the really bad effects of the Covid-19 disease. In one study, there was a stark difference between those who had less than 20 ng/mL serum levels, and those who had more. There are many studies which corroborate this, generally.

There was another study that showed that taking 5000 IU or 10000 IU would result in levels well below 210 ng/ml in average people, which is around the level where we'd expect side effects. This is not medical advice. I am not a doctor, just a random guy on the Internet. However, I take 5000 IU daily, and I intend to raise this to 10000 IU when I get vaccine doses. Again, this is not medical advice. Look up the studies yourself, and do your own research.

EDIT: negatively correlated