The Huck Institutes of the Life Sciences

Some vaccines support evolution of more-virulent viruses

Scientific experiments with the herpesvirus that causes Marek's disease in poultry have confirmed, for the first time, the highly controversial theory that some vaccines could allow more virulent versions of a virus to survive, putting unvaccinated individuals at greater risk of severe illness.
http://news.psu.edu/story/363898/2015/07/27/research/some-vaccines-support-evolution-more-virulent-viruses