I have seen scientific evidence that Vit D supplimenting helps. In Spain its prescribed if you get covid. re: "unless you are deficient": you supliment to ensure you arent deficient, you are foolish to risk zinc or Vit D deficiency if you know you have covid. If you plan a fews days in bed you are (usually) not going to get much sun. It's foolish to risk deficiency of any vitamin while ill.