It's going to mutate regardless of how many people are vaccinated. The entire point is to make it so that people don't have the runaway immune system responses that are melting lungs and brains.
Considering cross-immunization between variants with natural immunity, it might be more efficient to refrain from vaccines to give cross-immunity with a weaker variant.
https://www.washingtonpost.com/health/2021/03/11/immunocompr...
1. Likelihood of breakthrough infections.
2. Mutation rate.
3. Initial volume of infections.
For instance, if the likelihood of breakthrough infections is low enough to overcome the reproduction rate, the mutation rate is slow enough, and the initial volume of infections low enough, you'd expect that the virus would die out if enough people have had the vaccine, right?
On the other hand, if the likelihood of breakthrough infections were high, mutation rate were high, initial volume of infections high, makes sense that you would expect to see ongoing mutation.
Another way I think this might go is that mutations happen & breakthrough infections happen, but the breakthrough infections further boost immunity, and the severity never re-approaches the original severity of covid.
I'm gladly corrected if I missed something!
Even if that variant is more lethal (from which you might expect a smaller R0), it might be prevalent because of an augmentation in R0 from vaccination (which might be lesser had people developed a more complete natural immunity).