This recently published study suggests vitD supplements alone provide some protection, even for acute cases. It’s a small sample population (n=75) and a pre-print, but it looks convincing to me (non-scientist, non-medical professional):

“Of 50 patients treated with calcifediol, one required admission to the ICU (2%), while of 26 untreated patients, 13 required admission... Of the patients treated with calcifediol, none died, and all were discharged, without complications. The 13 patients not treated with calcifediol, who were not admitted to the ICU, were discharged. Of the 13 patients admitted to the ICU, two died and the remaining 11 were discharged.”

https://www.sciencedirect.com/science/article/pii/S096007602...

Previous HN discussion: https://news.ycombinator.com/item?id=24390451