Do you have the source for this because I'm genuinely curious. I've only gotten really sick once in my whole life, and I'd been in school and working with money for years before and while I'm not in school anymore I still work with money, and that one time was the same year I got the flu shot. It was a couple years ago so I don't remember how long after the shot that I got so sick.
And to Yaezakura, I appreciate the concern but since I'm not especially adventurous and keep away from wild animals and stay on trails while hiking, I can't say I'm especially concerned and would rather run the low risk of an encounter with something like that than ensure I get even a little. I've talked to a few hikers I trust and none of them get vaccinated so I'll take their word and experience rather than speculation from folks online.
Google "Does the flu shot give you the flu" and you'll have like 10 articles right there