A former CDC director (Tom Frieden), vitamin D may be the best way you can boost your immunity. He suggests sunshine or a supplement if you live in northern climes.
Of course, dose according to your physician’s direction and your current levels. If you are low, you should plan to take a hefty dose for a month or two. I know some physicians that prescribed 10,000 iu for a couple months. It takes quite a bit to right those levels if you are sub-clinical. But again, check with your doctor–hopefully you have an idea of your levels.
If not, take no more than 2,000 iu for a bit until you can get better advice.
Better yet, get it from the sun if you can. Without sun screen stopping short of sun burn. It works much better than pills.