The risk is certainly not zero but the calculation isn't that simple because the underlying mechanism isn't random. Vaccines work by training your immune system and don't persist in your body; for long-term concerns what we're really looking for is evidence that there's a previously-unknown time delay mechanism for immune responses or evidence of something specific to a disease such as the way Dengue fever can be more severe on a second infection (which is why the vaccine is restricted to people with a prior laboratory-confirmed infection).

Again, I'm not saying that there's no chance of a problem but by now a billion people have been vaccinated and a significant number of people are 18+ months out, with no indication that any of these hypothetical scenarios are actually happening.