Flu shots are pointless. I've gotten two in my life and I guess they got guess wrong for the strain twice because I had the flu hella bad both years I got inoculated.
I've been on 5,000 IU of Vitamin D for 15 years now and haven't had a flu or common cold for those 15 years. I've only missed a single day of work over that time and that was due to some rancid chicken vindaloo from a dodgy dot shop.
Dr. John Cannell is the guy you want to read for Vitamin D research. Vitamin D is the the most powerful steroid hormone known to mankind and its importance is far from being understood.
https://www.youtube.com/watch?v=--NqqB2nhBE