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 development of future vaccines, scientists said.

"The concern now is about the next-generation vaccines," said Penn State University's Andrew Read, also a co-author.

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

Other scientists cautioned against using the research to fuel anti-vaccination campaigns, as have been seen in the US recently.

"It’s important not to interpret this study as an argument against vaccination of our children against flu or any other disease," said Peter Openshaw from Imperial College in London.

"The standard vaccines that are in current use are safe and effective, and not prone to cause the emergence of more dangerous strains of viruses."

After disappearing from the US, measles reappeared in December thanks to some parents not having their children vaccinated.