It’s more that the claims are very confidently and loudly made by the same people who were wrong about the severity and spread of the virus, possible treatments, and made provably false claims about the vaccines, often for financial or political profit.

It’s certainly possible that there will be some long-term problem that only becomes apparent after a considerable period of time but there isn’t a known mechanism for that, and if that is identified it’ll be due to the actual scientists who are carefully studying it rather than the people trying the old conman’s trick of making enough vague claims that they’ll be able to say they predicted the future.