[2018] Vaccines Are Pushing Pathogens to Evolve

The team found that, over the course of their lives, the unvaccinated birds shed far more of the least virulent strains into the environment, whereas the vaccinated birds shed far more of the most virulent strains. The findings suggest that the Marek’s vaccine encourages more dangerous viruses to proliferate. This increased virulence might then give the viruses the means to overcome birds’ vaccine-primed immune responses and sicken vaccinated flocks.

Vaccines Are Pushing Pathogens to Evolve | Quanta Magazine

Just as antibiotics have bred resistance in bacteria, vaccines can potentially lose their effectiveness over diseases they controlled. Researchers are working to head off the evolution of new threats.