Vitamin D is by far the best supplement to avoid being sick.
Numerous studies have shown that it is more potent than vaccines, remedies, drugs, in the prevention of diseases, especially virus like influenza.
It is cheap and good for health.
Take 5000 IU a day all the year long, and if you get sick, you will be sick 2 days, maybe 3 and that's all.
And taking such high doses if Vit C is not a good idea for your kidney.