Either they had covid 19 or some other coronavirus and the first dose triggered a reaction due to t-cell memory? I know so many people that had no reaction to the first and only some fatigue with the 2nd. I just had my first on Wednesday, this morning is the first time I have felt "normal", headache and fatigue the past couple of days. Can't wait for my second one, yuck...
My 84 year old grandmother with multiple comorbidities had no reaction to either of the doses.
I understand the older you are, the weaker your immune system is, but you would expect some sort of reaction on the second dose...