I do D3 also but experimental evidence of D supplementation conferring influenza immunity is really sketchy. Difficult to replicate, etc. But taking it during the winter in cloudy, higher latitude areas would seem to make sense on general principles.
D3 helps with things other than flu. I have BAD allergies, and as a result, any additional immune system attack tends to "get a running start". For YEARS (maybe 15), I would have four or five BAD colds (not flu), with subsequent sinus infections, antibiotics, and the whole nine yards EVERY YEAR.
I had been slowly ramping up my D3 intake working with my doctor and having blood tests. Was still having colds. Then (unknown to me) my wife switched to a higher dose D3 caplet (2X the previous dose), which jumped me from 5000 IU to 10000. Haven't had a cold since (about two years now).
Admittedly anecdotal data, but I can assure you that "I" am convinced.