Every single person who's immune system eradicates the virus causes evolutionary pressure.
If your immune system takes a long time to do this, because eg. you didn't have a vaccine so it hasn't got an antibody template to work from, then the virus has much longer to evolve in the host, under a bunch of weird and wonderful new types of antibodies that get thrown against it.
If there's already vaccine exposure then the body just needs to ramp up the antibody production, which is much faster and will result in total virus eradication to each virus as the antibodies encounter them, not halfway-measures which might leave half-functioning viruses which might self-repair in mutagenic ways.
If your immune system takes a long time to do this, because eg. you didn't have a vaccine so it hasn't got an antibody template to work from, then the virus has much longer to evolve in the host, under a bunch of weird and wonderful new types of antibodies that get thrown against it.
If there's already vaccine exposure then the body just needs to ramp up the antibody production, which is much faster and will result in total virus eradication to each virus as the antibodies encounter them, not halfway-measures which might leave half-functioning viruses which might self-repair in mutagenic ways.