The mutations don’t change, the selection is what changes.
Poor or “leaky” vaccines allow replication and infection to occur, but suppress the original variant.
Absent the vaccine the original variant tends to win out for the same reason it was dominant to begin with.
Keep in mind that a more lethal variant of a virus tends to spread more slowly because it makes its host sicker and kills minimizing the social interactions that spread the virus. Less dangerous variants spread faster.
If you get a vaccine that works by minimizing symptoms rather than infection, a lethal variant can spread as well as a non-lethal variant.
You might think, “no problem, I just need to get vaccinated.”. Actually, that is a far worse problem — you make the human race dependent on that vaccine for it’s survival. That’s the case with Marek’s disease in chickens.
And these opinions of yours are based on?