It won't stop it, but will decrease such a likelihood by many orders of magnitude. Every single covid case has a certain likelihood of mutation. Now if you have 1 billion cases or 1 thousand cases, this will make this difference in the likelihood of the event to appear. Same with transmission, if your body starts fighting the virus from day one, you will replicate less virus and will be less contagious. You might still infect someone though, how much less likely is this event is the question to be answered.