Off the top of my head, I can think of a half dozen instances when respiratory disease prophylaxis (adenovirus, flu, Bicillin) was stopped and outbreaks hit right on cue. Lost training and readiness, hospitalizations, sometimes even deaths. Preventable.
Second bonus: Flu is less common in the off season but it does still circulate. It only takes one person to bring it in. Once introduced, it'll get around reaalllly easily.
I used to do respiratory disease surveillance as an epidemiologist for the Army. Trainees at Basic come from all over the country, bringing local germs with them. They live, train and sleep in close quarters, under huge physical strain. Training camps are famous as outbreak hotspots. 1/2
Bonus: I did some research once and decided the program I managed was likely the longest continuous disease surveillance program in the country, going back at least 50 years (now more). Of respiratory disease in basic trainees! That's how fundamental it is.
I used to be a civilian epidemiologist for the Army. The optional-flu-vaccine policy won't last.