Your seasonal FLU shot, would be my guess.
Every year, you are being told to get it to …. blah blah blah.
The annual flu vaccine doesn’t require boosters. If you’re in a “At risk” category then you need to take it annually
There is no “need” to take flu jabs annually, everyone has the choice whether to take it or not, and if it works for you fair enough
In my case, every year i did take the flu jab i’d get fever (not that a fever on it’s own is gonna knock me out), but also 2 weeks of a real bad sore throat and 10x more snot than what’s normally in my nose, and the latter is a massive pain in the ass, now the last couple years i didn’t take the flu jab, i went 1 year with no flu whatsoever, and this last year with the mildest flu i’ve ever had, 2 days of sneezing and feeling colder than usual, cured by wrapping myself in a blanket and binging anime for those 2 days!