> imperfect vaccines can cause selective pressure which enhances the fitness of highly virulent pathogens
The number of ways in which we undertaking a natural experiment in viral evolution has been top-of-mind for me as well. But the greater issue in my view is not that the vaccines are imperfect. All vaccines are somewhat imperfect. On its own, I the mRNA vaccines were probably entirely adequate.
The problem is that the human population, as a result of policy decisions, will continue to have an immediately-adjacent reservoir of infection [0] with whom we are constantly in contact. I speak, of course, of the large numbers of unvaccinated among us. With vaccinated-to-vaccinated chains of transmission, there's a greater chance that a chain of vaccine-resistant variant will die out because the vaccine both reduces the risk of becoming infected [1] and the risk of transmitting infection [2].
But in a heterogeneous population, with large numbers of unvaccinated individuals, those chains of transmission can go on indefinitely, hopping back-and-forth between the two populations. That's a far greater risk here than the issue of a "leaky" vaccine. A vaccine might "leak" a little bit, yet the probabilities of infection and subsequent infection remain so low that all chains of transmission quickly terminate. Instead, we have a situation where the virus can slip into a separate (reservoir) population where it continues unchecked.
That, in my view, is the real risk to furthering infection.
[0] https://en.wikipedia.org/wiki/Natural_reservoir
[1] https://www.cdc.gov/media/releases/2021/p0607-mrna-reduce-ri...
[2] https://sporevidencealliance.ca/wp-content/uploads/2021/06/T...