Study confirms theory that ‘leaky’ vaccines support evolution of dangerous viruses

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Researchers have confirmed for the first time ever that ‘leaky’ or ‘imperfect’ vaccines enable viruses to evolve into more dangerous variants that put unvaccinated individuals at greater risk of more severe versions of illness.

Researchers carried out experiments with the herpes virus strain that causes Marek’s disease in poultry and their findings have important implications for food-chain security and food-chain economics, as well as for other diseases that affect humans and agricultural animals.

Imperfect, or leaky, vaccines are those which prevent the vaccinated host from getting sick but are not capable of preventing the transmission of the virus, thus the virus is able to survive and to spread throughout a population. Perfect vaccines are those that not only protect the vaccinated individuals, but also prevent the transmission of virus to others.

Published in Open Access journal PLOS Biology, the study investigates how the use of “leaky” or “imperfect” vaccines can influence the evolution of virulence in viruses. The study is a collaborative effort of an international group of researchers led by Professor Andrew Read, the Evan Pugh Professor of Biology and Entomology and Eberly Professor in Biotechnology at Penn State University, USA and Prof. Venugopal Nair, the Head of the Avian Viral Diseases programme at The Pirbright Institute, UK.

Researchers, in their tests of the leaky Marek’s disease virus, found that unvaccinated chickens died while their vaccinated counterparts survived, but transmitted the virus to other birds left in contact.

Researchers say that their study demonstrates that the use of leaky vaccines can promote the evolution of nastier ‘hot’ viral strains that put unvaccinated individuals at greater risk.

Read said that the vaccines are classified as “perfect” because they are designed to mimic the strong immunity that humans naturally develop after having been exposed to one of these diseases. He added that their researchers demonstrates that another vaccine type allows extremely virulent forms of a virus to survive and allows the virulent virus to continue evolving precisely because they allow the vaccinated individuals, and therefore themselves, to survive.

Researchers expressed concerns over the next-generation of vaccines as if they turn out to be leaky, they could drive the evolution of more-virulent strains of the virus. Researchers say that there is an urgent need to determine as quickly as possible that the Ebola vaccines that now are in clinical trials are not leaky and that they are able to prevent the transmission of the Ebola virus among people.

“We do not want the evolution of viral diseases as deadly as Ebola evolving in the direction that our research has demonstrated is possible with less-than-perfect, leaky vaccines,” Read said.

He also recommends vaccination for individual protection. “When evolution toward more-virulent virus strains takes place as a result of vaccination practices, it is the unvaccinated individuals who are at the greatest risk. Those who are not vaccinated will be exposed, without any protection, to the hottest strains of a virus. Our research provides strong evidence for the importance of getting vaccinated.”