Even better than giving vitamin D to patients already COVID-stricken is for people to take it themselves before they encounter COVID. Especially in winter--or quarantine.
And one more piece of advice applies (although not to immunity): When you take vitamin D3, make sure it is accompanied by vitamin K2. The K2 directs the calcium mobilized by vitamin D to go to the right places (ie, bone rather than blood vessels). You can find the twain in one palatable pill.