'Leaky' vaccines could make diseases more deadly: study

More harmful viruses can evolve from the use of so-called "leaky" vaccines, researchers, including one of Indian-origin, have confirmed for the first time.
Scientific experiments with the herpesvirus such as the one that causes Marek's disease in poultry have confirmed 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 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, an author of the paper published in the journal PLoS Biology, from the 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 a type that is "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 UK where the experimental work was carried out.

"These vaccines also allow the virulent virus to continue evolving precisely because they allow the vaccinated individuals, and therefore themselves, to survive," said Nair, who is the head of the Avian Viral Diseases programme at the Pirbright Institute in UK.

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.
"In our tests of the leaky Marek's-disease virus in groups of vaccinated and unvaccinated chickens, the unvaccinated died while those that were vaccinated survived and transmitted the virus to other birds left in contact with them," Nair said.

"Our research demonstrates that the use of leaky vaccines can promote the evolution of nastier 'hot' viral strains that put unvaccinated individuals at greater risk," he said.
The World Health Organisation recently reported laboratory-confirmed cases in China of human infection with the avian influenza virus, researchers said.
"We humans never have experienced any contagious disease that kills as many unvaccinated hosts as these poultry viruses can, but we now are entering an era when we are starting to develop next-generation vaccines that are leaky because they are for diseases that do not do a good job of producing strong natural immunity - diseases like HIV and malaria," Read added.