Some vaccines support evolution of more-virulent viruses

Scientific experiments with the herpesvirus such as the one 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. The research has important implications for food-chain security and food-chain economics, as well as for other diseases that affect humans and agricultural animals.