Andrew F. Read of Penn State is one of the more outspoken voices in the field of evolutionary medicine, an emerging specialty that urges medical researchers to account for the forces of evolution in their work. Some of Read’s own research, for instance, looks at the growing problem of antibiotic resistance. Antibiotics kill some germs but allow others to survive, reproduce and emerge as new drug-resistant strains (“Dr. Darwin,” Winter 2015). That sequence of events is a classic exhibition of natural selection at work.

In a controversial 2001 paper in Nature, Read applied similar thinking to vaccines, which work by training the immune system to identify and destroy a specific pathogen. If a vaccine is only partially effective, he argued, it may protect the recipient from disease but allow the pathogen to replicate in the host’s body, which could then spread to unvaccinated populations. At the time, the idea that such evolution could be driven by “leaky” vaccines was only theoretical. But in a paper published last July in PLoS Biology, Read and several colleagues said they have confirmed the idea in a study involving Rhode Island Red chickens.

Read’s group looked at whether a vaccine for Marek’s disease, a condition specific to chickens, created an evolutionary condition that enabled virulent strains to thrive. They infected different groups of chicks—some that had been vaccinated and some that hadn’t—with the most lethal strains of Marek’s disease. Unvaccinated birds died within 10 days. Most vaccinated chicks, meanwhile, survived the infection for 30 days or more. But because the vaccines didn’t make them virus-free, the vaccinated chicks transmitted a great deal of lethal germs through shed feathers, which quickly infected and killed unvaccinated chickens nearby. The experiment suggests that a leaky vaccine can cause recipients to become incubators and carriers of dangerous strains.

This phenomenon, however, has never been observed in humans, says pediatrician Paul Offit, director of the Vaccine Education Center at The Children’s Hospital of Philadelphia. Offit is co-inventor of one of two rotavirus vaccines now available that prevent severe future bouts of this gastrointestinal disease but don’t provide lifelong immunity. “We’ve had a dramatic reduction in the instance of rotavirus and we haven’t seen evidence of more virulent strains emerging,” Offit says of his vaccine. “It’s been almost 10 years, and nothing.”

Read stresses that vaccines for common diseases such as measles, mumps and polio don’t leak, since they mimic natural immunity, which lasts a lifetime. (Why others do leak is harder to say, he adds.) And Read is quick to note that he is strongly in favor of immunization and the advantages it confers to those vaccinated. Those who falsely link childhood vaccinations to autism and other diseases have dampened legitimate conversations, he says. “The fear of risks in the public is leading to a fear of discussion about the risks [of vaccines] in the scientific community,” says Read, “and I think that’s bad all around.”