All I know is after my second shot I felt like crap for four days. They told me maybe one. Nope, four days. Had a low fever. My lymph nodes in my arm pit were swollen. Talking to my daughter in the medical field she said it was actually a good reaction. It shows my body went to war with what it perceived to be covid. Now it knows how to fight the real thing. She said statically younger people have more severe reactions than older people. She said so many old people get the shot and never complain of anything like a fever or being run down.
I could see how some people getting the shot could have problems.