It could have been an allergy to one of the ingredients. They have been refining and simplifying vaccines over the years in light of new research. I have a mild allergy to neomycin, which is a common ingredient in the flu shot. The newer cell-based version typically does not contain it. The tech though I was vax-hesitant when I insisted on seeing the covid shot ingredient list.
I don't think it was an allergy, usually an allergy has a fast reaction and happens within 15m, I usually get an immune response though which is more similar to what happened with second shots of the mrna vax for covid. I get a high fever for 3 days and that's that. But the last time I had the shot was after I bludgeoned my foot with a rusty shovel that was about 4 years ago and I had no immune response this time which is more akin to what happens when I get a flu shot.
