My ONLY flu shot was in 72 while in the USAF.
I got my only one in the AF in 66 as a boot. Several days later I had the worst flu I have ever had. I’ve not taken one of those things since. Nowadays I and many others I have proselytized take the D-3 supps through the winter or me, when I work at night, all year. We do NOT get flu or any colds.