Is it reasonable to believe that a lot of these risks won’t become visible before year or two has been passed?

In all three cases I mentioned, as far as I know there was either immediate indication or suspicion raised within the 1st year, but took several years to reach conclusions.

The polio case was very clear very quickly, the others not so much.

There's quite a bit about long term autoimmune response that we don't understand. I'm not up-to-date on everything, but from what I remember, strep-A infection is associated with increased likelihood of RA decades later; Epstein Barr exposure (causing Giullan Barre in children and Mono in adults) is associated with higher probability for MS, and there are a few other such suspected causalities - autoimmune diseases are no joke, and it seems like they can be triggered by either a pathogen (virus, bacteria) or a vaccine (which is designed to provoke the same kind of immune reaction without letting the pathogen do harm).

On average to society, it is almost sure that if nothing becomes suspicious within a year, it is a net gain.

To individuals, the question is much more complicated - autoimmune diseases often appear in clusters; I think it is prudent for people with a history (or even family history) of autoimmune disease to hold off as much as they can - as it may take years or decades to figure out how it effects the unlucky ones (of which they are much more likely to be).