Lead poisoning for example takes years to manifest itself.
So did other drugs which took years until regulators retracted them because they did more harm then good.
Specifically about mRNA vaccines, we know little about the long term effects of the lipid nano particles used in them and there are reasons to be concerned.
> In 2019, Magnus Carlsen and Anish Giri – who as of July were the number 1 and number 10 players in the world, respectively – promoted a #MoveforEquality campaign as a way of acknowledging social inequalities. In their game, black moved first and the line was, “We broke a rule in chess today, to change minds tomorrow.” It was billed as an anti-racist statement, but some took it as a suggestion to change the rules of chess to black having the first move.