I’m supplementing with Vitamin D and have been for a couple of years.
i just take it if i feel a cold or flu coming on (or when really low in D)- take the ‘vit D hammer’- and it ‘seems to work’ pretty well- but then again- maybe i wasn’t getting actual flu and just a mild cold and only thought the D helped reduce a flu- who knows? but can’t hurt- D is needed, and usually I’m low in it- like really low- one reading i was 8