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. 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.
“The challenge for the future is to identify other vaccines that also might allow more virulent versions of a virus to survive and possibly to become even more harmful,” said Andrew Read, a leader of the research team whose paper describing the research will be published in the July 27, 2015 issue of the scientific journal PLoS Biology. Read is the Evan Pugh Professor of Biology and Entomology and Eberly Professor in Biotechnology at Penn State University.
“When a vaccine works perfectly, as do the childhood vaccines for smallpox, polio, mumps, rubella, and measles, it prevents vaccinated individuals from being sickened by the disease, and it also prevents them from transmitting the virus to others,” Read said. These vaccines are dubbed “perfect” because they are designed to mimic the perfect immunity that humans naturally develop after having survived one of these diseases.
“Our research demonstrates that another vaccine type allows extremely virulent forms of a virus to survive — like the one for Marek’s disease in poultry, against which the poultry industry is heavily reliant on vaccination for disease control,” said Venugopal Nair, who led the research team in the United Kingdom where the experimental work related to this study was carried out. Nair is the head of the Avian Viral Diseases program at the Pirbright Institute, which also hosts the OIE Reference Laboratory on Marek’s disease. “These vaccines also allow the virulent virus to continue evolving precisely because they allow the vaccinated individuals, and therefore themselves, to survive, Nair said.
Less-than-perfect vaccines create a ‘leaky’ barrier against the virus, so vaccinated individuals sometimes do get sick, but typically with less-virulent symptoms. Because the vaccinated individuals survive long enough to transmit the virus to others, the virus also is able to survive and to spread throughout a population.