Defective or 'leaky' vaccines may lead to even more powerful viruses, according to a study on poultry that raises concerns about vaccine development in humans. When a vaccine works as intended - such as for smallpox, polio and measles - it protects those vaccinated and prevents the transmission of the virus.
But the study, published in PLOS Biology, reported that imperfect vaccines shielded poultry but also allowed the virus to survive in an even more pernicious form.
"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," said co-author Venugopal Nair of the Pirbright Institute in the UK.
"These vaccines ... allow the virulent virus to continue evolving," he said.
The researchers did not claim that the vaccine was directly responsible for increasing virus strength. The process, they said, was not as clear as the evolution of germs that develop resistance to antibiotics.
But the study did show an unmistakable correlation between vaccination and the development of Marek virus strains that changed from being harmful to deadly for some livestock. Current human vaccines are not in doubt, but the findings raise questions about the developement of future vaccines, scientists said.
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