IDK, I didn't even feel the first shot, and it got sore.poor atty
i think that arm soreness relates directly to the skill of the vaccine giver.
This 2nd shot, I felt a minor jab, but otherwise it was fine, and it's in the same arm (my choice) so that spot is expected to be more tender this time.
This guy has given 1000's of shots. He's good at it by now.