Some vaccines support evolution of more-virulent viruses

Any vaccine against any rapidly mutating virus is "leaky". The classic exam ples are the various vaccines against the various (and rapidly evolving) st rains of seasonal influenza. They don't seem to have made influenza more de adly, and we have been using them for years now..

amples are the various vaccines against the various (and rapidly evolving) strains of seasonal influenza. They don't seem to have made influenza more deadly, and we have been using them for years now..

Just to be clear, the term "virulent" has two meanings (at least). One is to be severe or harmful while the other is simply to be infectious. So a m ore infectious strain could apply without being more deadly. It just depen ds on what Humpty Dumpty meant when the term was used, no more and no less.