About this, I actually have had some hesitancy as somebody who's received both vaccination doses so far when it comes to getting a booster shot.
Heart inflammation is a possible long-term side effect, and while it's exceedingly rare (724 confirmed cases from Pfizer), the fear is that it might become more pronounced with the booster shots. And for me, my father and I both have genetically weak hearts which are predisposed to becoming inflamed. I've actually been following a diet intended to reduce the rate of that happening/taking a supplement regime, and I'm going to be going back in to check with my doctor and see if it's working out. I'm not saying I'm perfect considering how lethargic I was at the start of the pandemic - I definitely don't have as much stamina as I used to, and I've lost almost 40 pounds on this diet already - but there's a very real chance I might have to take statins for the rest of my life as an otherwise healthy man under 30 years old.
I'm not saying that the odds aren't slim. But they're there, and they might be increasing. And it's difficult for me to bring myself to take a vaccine which might make my current condition worse. I hope you can understand that for some, we're very much caught between a rock and a hard place when it comes to our health and possible side effects from the vaccine.