People have reactions to alot of vaccines. The yearly flu shot usually gives me a mild semi-cold for a day. Tetanus shots always make my arm feel like I was punched by bigfoot. I had to get hepatitis vaccines (thanks gross NJ restaurant) and it made me nauseous for 2 days after the second dose.
But I'd much rather have the sniffles, a sore arm, or a tummy ache than the flu, tetanus, or hepatitis. Same with covid. My buddy just had it and was on his ass for over a week. Bad cough, high fever, said he was so sore he could barely move and he is in his 30's and good health. I'll take a day or 2 of feeling icky from the shot over that...
I'd be curious to know which vaccine made people feel what though. My mom had the moderna and she got bad cold like symptoms for 2 days. My dad also had the moderna and had a cold and the squirts for a couple days. I had the pfizer one and my side effects were very mild. Just sniffles and the poops for like 8 hours.
That's the only downside, you don't get to choose what vaccine you get and don't know what they have until you get there.