In other words, a so-called "leaky vaccine" or a vaccine that lowers the chances of severe illness, but still allows for transmission of a virus, can create conditions that promote the evolution of more virulent pathogens.
For further reading here is a 2015 NIH-funded study that found "imperfect vaccination can enhance the transmission of highly virulent pathogens.". https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516275/
The chart entitled "genomic epidemiology of novel coronavirus" shows the increase in mutations after the introduction of these leaky vaccines at the beginning of the year.
Makes sense. Think of it from a bacteria / antibiotic perspective. It's why doctors and vets will stress taking the entire course of antibiotics they give you or your pet. You need to wipe everything out or you wind up creating evolutionary variants that are antibiotic resistant.
Same reason they're being far more judicious with giving antibiotics these days.
Introduce a "vaccine" with an extremely low efficacy and it's going to hinder reaching herd immunity and knocking out the virus.