I think going out in the sun mid-morning for 30 minutes can help also. In most locations but of course some folks need that supplement. Good pointsIt's worth noting that the body's need for Vitamin C rises massively when fighting an infection since Vitamin C is kind of like the body's version of "ammunition" for the immune system "gun". Also, it's almost impossible for most people to maintain D levels above 50 (where the evidence seems to say they should be for optimal immune function) without supplementing D and K2.