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updated 7/31/2015 10:45:34 AM ET 2015-07-31T14:45:34

Some vaccines may cause viruses to evolve into deadlier forms, a new study suggests.

Chickens that are vaccinated against a strain of Herpes known as Marek's virus may allow the unvaccinated birds in the flock to catch a deadlier version of the same virus.

The effect has so far been demonstrated with just one bird virus, though it's possible it may also occur with some human vaccines, the researchers said. Still, among humans, the evolution of the virus into a deadlier strain would only be possible in cases where vaccinated people can still spread a virus even if they show no symptoms of disease, as is the case for some newer human vaccines, such as whooping cough.

But that doesn't mean you should forgo the jab at your doctor's office ; if anything, the new results provide even more reason for people to get vaccinated, said study co-author Andrew Read, a disease ecologist at Pennsylvania State University in State College. After all, if a human vaccine lets deadlier versions of a disease flourish, that is all the more reason to be protected from those deadly strains, he added. [ 5 Dangerous Vaccine Myths ]

Long-standing theory

Read and his colleagues crunched the numbers several years ago and found that in some circumstances, certain types of animal vaccines could lead to the evolution of more virulent strains of disease. But the idea was controversial and no one knew whether it was more than a theory.

The team looked for the evolution of viruses by studying the course of a particular disease known as Marek's virus, which has a vaccine. In its usual form, Marek's disease may take about two months to kill a chicken, causing numerous lesions to grow in the animal's body and causing one-sided paralysis. But in its deadliest forms, the disease can cause unvaccinated chicks to quickly become disoriented and paralyzed, unable to feed or move. The most virulent strain can kill chicks in about a week, the researchers said. Usually, however, birds with the deadliest strains die so quickly they don't have time to spread the virus, so transmission is snuffed out quickly, Read said.

The vaccine against Marek's disease stops chicks from developing symptoms of the disease and allows laying hens to pass on a short-lived protection against the disease to their chicks. But even vaccinated chicks can still catch the virus and spread it when they shed their dander, if the virus is circulating in the flock, Read said. [ 6 Superbugs to Watch Out For ]

In a series of experiments, the team found that the vaccination of chicks led to the spread of "hotter strains" of the virus — deadlier versions that killed unvaccinated members of the flock more quickly, the researchers reported in the study, published Monday (July 27) in the journal PLOS Biology.

Here's how it works: Vaccinated chickens exposed to deadlier strains still shed the virus in their dander, spreading it to unvaccinated chicks in their flock. Normally, the deadly strains would have killed the chicks before they could pass it on, but the vaccinated chicks created the perfect source for the transmission of deadlier strains, the researchers found.

Leaky versus perfect vaccines

The new findings are fascinating, but they have only been demonstrated with one particular disease in animals, said Joanne Devlin, a veterinary virologist at the University of Melbourne in Australia, who was not involved in the study.

"I'd be wary of extrapolating too far into other systems," Devlin told Live Science. "We use vaccines a lot differently in humans than we do in animals, and the vaccines themselves are different."

For instance, animal vaccines are largely developed for agricultural purposes, so people don't care too much about saving every single animal or stopping all symptoms of disease; they just want to stop outbreaks to protect their bottom line. As a result, most animal vaccines are developed rapidly, and undergo fewer tests to show safety and effectiveness than those that are required for human vaccines. There are also more animal vaccines in use, Devlin said.

Many animal vaccines are known as leaky vaccines, which means they reduce symptoms but people can still be infected by the viruses. Most human vaccines, such as for polio, and the measles, mumps and rubella (MMR) vaccine are perfect vaccines, so they also stop transmission of the disease in question, Read said. (It's not clear why some vaccines are leaky, but diseases where the body does a good job of creating lifelong immunity after a single exposure, such as for childhood diseases like measles, seem to be more suited to the development of perfect vaccines, Read added.)

"Diseases for which natural immunity is not good — we have real trouble building these vaccines," Read told Live Science. "That's because natural immunity is quite leaky, and we have trouble generating anything that's better than nature."

Still, it's at least possible that the same effect could be seen in humans. Some of the newest vaccines — notably whooping cough, malaria and HIV vaccines — are leaky, Read said.

What's "not clear is how far this result generalizes — if it did generalize to most other vaccines, there could be serious ramifications," said James Bull, a biologist at the University of Texas at Austin, who was not involved in the current study.

For instance, it would be extremely important to know if Ebola vaccines could lead to such transmission, Read said. Follow-up studies should test whether leaky human vaccines could also lead to the spread of deadlier disease strains, he added.

© 2012 LiveScience.com. All rights reserved.

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