In my very large family (ages 20 to 101 y/o) almost everyone who has had the Moderna vaccine has said they felt like they had the flu for about 24 hours after the 2nd dose. For those who had the Pfizer vaccine, side effects were pretty much just a little pain at the injection site. 101 y/o grandma was very nervous about getting vaccinated, but her MD son-in-law convinced her to get it. She got Pfizer and had no side effects other than pain at the injection site.
Also, I imagine that genetics and health plays a part in who gets sick from the shot.
I know people in their 90s who got covid and lived.
I don't plan on taking the shots, but believe in protecting my self with known prophylactics.
I read the details on the 1976 swine flu vaccine campaign and also on the original polio vaccine campaign in use before we switched to the Sabin vaccine.