Mutations evolve in infected people. Vaccines reduce infection, both directly and via reduced spread. Hence fewer mutations. But muations happen all the time.

Mutations evolve in infected people. Vaccines reduce infection, both directly and via reduced spread. Hence fewer mutations. But muations happen all the time. Is a particular mutation viable, does it spread, then outcompete? Thankfully emperical evidence shows thr neutralizing antibodies response, due to vaccination, reduces spread of COVID.