For all we know, mRNA vaccines can alter your gametes. We don't know because this is the first time we ever use them. They aren't like viruses or weakened viruses. They are just a massive injection of genetic material (much of it deformed) that encodes a single foreign protein. No human being has ever had such a thing done to them until very recently. mRNA vaccines are not part of any natural process that modern humans or our ancestors have had to withstand to get to the present day.