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Imperfect vaccines can help viruses become stronger

According to a study in PLOS Biology, live Marek's disease virus (MDV) vaccine can lengthen the disease transmission period, allowing stronger strains of the virus to reach unvaccinated birds.

Cody Griffin, Rapid News Network, Jul 28, 2015

And the practice has been successful for many years, with "perfect" vaccines protecting children against polio, mumps, measles or smallpox, but it's the "imperfect" or, otherwise called "leaky", vaccines that might pose a threat for future development.

Although the study did not show that vaccination caused MDV to mutate into more virulent forms, Nair said that the prolonged infection period within immunized chickens could give the virus more time to morph.

According to a study published today (July 27) in PLOS Biology, inoculating chickens with a live Marek's disease virus (MDV) vaccine can lengthen the disease transmission period, allowing stronger strains of the virus to reach unvaccinated birds, effectively increasing the virus's fitness.

After experiments done in a specialized pathogen-containment facility at The Pirbright Institute in the United Kingdom, the researchers concluded that the vaccines developed to combat Marek's disease were imperfect or leaky.

Less-than-perfect vaccines create a "leaky" barrier against the virus, so vaccinated individuals sometimes do get sick, but typically with less-virulent symptoms.

British investigator Professor Venugopal Nair, from The Pirbright Institute near Woking, Surrey, 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".

"These vaccines... allow the virulent virus to continue evolving", he said. For now, this so-called leakiness only exists in vaccines used to treat farm animals.

New vaccines can spread killer diseases, say experts. Since its first discovery, in the 1950's, it was known to be a minor affliction that didn't cause much harm, but has now managed to transform in a much more virulent virus that is capable of killing entire flocks within 10 days. That has led to a pile of theories regarding cases such as the avian influenza.

During the avian flu outbreak, infected poultry infected in the US and Europe were culled, but farmers in south-east Asia relied on "leaky" vaccines. "If the next-generation vaccines are leaky, they could drive the evolution of more-virulent strains of the virus".

"In our tests of the leaky Marek's disease-virus in groups of vaccinated and unvaccinated chickens, those unvaccinated died while those that were vaccinated survived and transmitted the virus to other birds left in contact with them", said Venugopal Nair, who led the research team in Britain where the experimental work was carried out. In this day and age, we have a long list of "perfect" vaccines that work flawlessly to their objective and boost the health industry, so current vaccines are not in doubt.

"It's important not to interpret this as an argument against vaccination".

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