Vaccines are undoubtedly a valuable part of a healthy society. Without them, diseases like polio and smallpox would still be ravaging the population. Most people rightly see vaccines as a blessing that protect us from dangerous pathogens, but a new study has shown that some types of vaccines may have unintended consequences. And no, I’m not talking about autism.

I’m actually not even talking about humans yet, but research concerning farm animals, a contagious disease, and an imperfect vaccine is sparking thoughts about the way we create and use vaccinations. A study published in PLOS Biology looked at Marek’s disease — a herpes virus that infects chickens — and its vaccine. The vaccine being used to immunize chickens against Marek’s is an imperfect, also called “leaky,” vaccine, meaning that it does not make the host completely immune to the disease and incapable of spreading it to others.

Marek’s disease has always been highly contagious, but it has recently turned deadly. Chicken farmers are seeing increasingly virulent strains in their flocks. The vaccine they’re using does keep chickens from getting the disease, but it seems that unvaccinated chickens are getting sicker than ever.

The research team, headed by Andrew Read, the Evan Pugh Professor of Biology and Entomology and Eberly Professor in Biotechnology at Penn State University, and Venugopal Nair, head of the Avian Viral Diseases program at The Pirbright Institute, could not determine whether the vaccine caused more virulent strains to develop, but they knew for sure that the existence of the vaccine allows them to keep existing.

“Our data show that anti-disease vaccines that do not prevent transmission can create conditions that promote the emergence of pathogen strains that cause more severe disease in unvaccinated hosts,” they wrote in the study.

A vaccine that does not keep its host from spreading the disease allows super-virulent strains of the disease to continue because it allows that one chicken to continue living when it would have died very quickly without the vaccine. More time living means more time to spread the disease to other chickens, resulting in many more cases than a flock would have seen without the vaccine.

Read explained that this isn’t a big problem for the chicken industry at the moment, because it’s unimportant to get all birds on a farm vaccinated, and Marek’s only affects chickens.

"The problems would start if we weren't just talking about chickens," he told The Washington Post.

If a human were to come in contact with a particularly virulent strain of a disease transferred between animals and humans, say, avian flu, the problem becomes much scarier.

"It's just not possible to predict if a virus will get more or less nasty when it jumps species," Read said. "It's not predictable in general, and we just don't know how that works with avian flu. It's just not a good idea to create these conditions."

Researchers stressed that their findings had nothing to do with current human vaccines that are frequently attacked by anti-vaxxers. It’s likely, however, that future vaccines attempting to tackle intimidating diseases like HIV and Ebola will start out as imperfect ones. It is nearly impossible to build up a natural immunity to diseases such as this, and the study authors say that even an imperfect vaccine would be a medical breakthrough.

"To me, it doesn't mean that we shouldn't use imperfect vaccines," Read said. "Let's say we become certain that a malarial vaccine is going to drive the evolution of more dangerous malarial parasites. That just means we have to be aware of how to avoid transmission."

Read said higher vaccination rates would be an appropriate solution to the issue presented in the study—not the abandonment of vaccines.

Source: Read A, Baigent S, Powers C, Kgosana L, Blackwell L, Smith L. “Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens.” PLOS Biology. 2015.