I’ve never had a flu jab, so no idea.. I know two people who have had the Covid jab – one had the sore arm and felt a bit fluey for 24 hours, the other had no reaction at all. I wondered if having no reaction might indicate prior immunity as his wife and daughter have had Covid and he would have already have been exposed to the virus.
Perhaps the other way around, if you’ve had previous exposure to the Covid spike proteins etc., then your body would mount a much faster response to the vaccine with those same spike proteins.
Think back to BCG vaccinations, the test they did before vaccination, I was the only one in the class who reacted to the test because I’d already been vaccinated when I was 6 months old as my older sister had already had a mild case of TB.
However, if you’d had another coronavirus such as one of those that can cause a common cold, you’d also possibly get a faster more expressive reaction.
Everyone seems to react a little differently.