All that said, Vitamin D has been found to be associated with lower rates of influenza in children in a rigorous clinical trial. It is not a replacement for a vaccine, though. And consult a doctor for the correct dose: excessive doses can be toxic!
Also, Vitamin D is one vitamin that light skinned people can produce with no supplement at all — it is made in the skin when exposed to sunlight. We frum Jews don’t get a lot of sun exposure on our skin because we cover up so much, but at least expose your face to the outdoor sun a little more (but not enough to get sunburned) — it really is good for you.