Thats even an option? When I was active USAF, the flu shot was mandatory. They went out of their way to hunt down those who slide through the cracks to make us get it.

They usually gave the flu shot the week before Thanksgiving. One year, I took leave to visit family and wasn't on base to get the shot. But exposure to family got me sick shortly after I returned home. Then, after I recovered from that, the rest of my Flight, having got the flu shot, was now getting sick with the flu. I then got sick again from that. After recovering from that, they'd finishing tallying the numbers and sent me a summons to get the flu shot. Week later, I had the flu again. Pretty much spent the entire month of November and the first half of December with the flu.