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“It’s important that we pay close attention to the Ebola vaccine in the ongoing trials. We want to know if a person who has been vaccinated and comes in contact with Ebola, whether there is any virus replication in that person and whether that means there could be onward transmission,” Read said. “If those are leaky in humans, it would be potentially very disadvantageous as it could help establish an endemic.”

https://web.archive.org/web/20210924184314/https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous

"There is a theoretical expectation that some types of vaccines could prompt the evolution of more virulent (“hotter”) pathogens. This idea follows from the notion that natural selection removes pathogen strains that are so “hot” that they kill their hosts and, therefore, themselves. Vaccines that let the hosts survive but do not prevent the spread of the pathogen relax this selection, allowing the evolution of hotter pathogens to occur. This type of vaccine is often called a leaky vaccine. When vaccines prevent transmission, as is the case for nearly all vaccines used in humans, this type of evolution towards increased virulence is blocked. But when vaccines leak, allowing at least some pathogen transmission, they could create the ecological conditions that would allow hot strains to emerge and persist. This theory proved highly controversial when it was first proposed over a decade ago, but here we report experiments with Marek’s disease virus in poultry that show that modern commercial leaky vaccines can have precisely this effect: they allow the onward transmission of strains otherwise too lethal to persist. Thus, the use of leaky vaccines can facilitate the evolution of pathogen strains that put unvaccinated hosts at greater risk of severe disease."

https://web.archive.org/web/20210925174917/https://journals.plos.org/plosbiology/article/info:doi/10.1371/journal.pbio.1002198