I think the thread is worthy of a bump. Having read the information elsewhere, I have been taking 3,000 or 4,000 IU of D3 for a few days. The cost is low, the risk is small and the possible benefits are large (not just limited to flu prevention).
I have been talking 5,000 IU a day for two years. Blood work showed that I was very Vit D deficient.
Come to think of it, I have not had a cold or the flu during this time.