> This indicates that if the immune system is primed to identify a certain variant, its performance is hindered as opposed to a naive immune system.

This does not mean that the performance is hindered. It just means that there is a larger immune system performance differential.

Here is how it might work.

The regular COVID-19 is highly prevalent, the variant has much less prevalent. The vaccine is highly effective against regular COVID-19. Let's also say that it is somewhat effective against the variant so the performance of the vaccinated immune system is actually better than the naive immune system.

Because of the selection effect, you could have a higher variant prevalence compared to regular COVID-19 in the vaccinated population, even though the vaccine actually improves the peformance of the immune system with regards to the variant.