In the 1960s a cancer-causing herpes virus was ravaging the poultry industry. The virus caused what's called Marek's disease—and killed 1 to 2 percent of the birds. "Given that there are billions of birds in the industry, that's a lot of birds." Andrew Read, an evolutionary biologist at Penn State University. He says the virus was easy to catch. "The dander of chickens if full of the virus. If you shake a chicken, the virus drops out."

Then, in 1970 a new vaccine put an end to most of the deaths. But the poultry vaccine, unlike most, was a so-called "leaky" or "imperfect" vaccine. "The vaccine is lifesaving, but it allows the infection to persist and transmit from the host." Meaning you could still shake a vaccinated chicken—and make it rain viruses.

Now, Read and his colleagues have shown that these leaky vaccines may actually give some viral strains an evolutionary leg up. Because the most virulent strains usually wipe out unvaccinated birds in just 10 days—not enough time for the birds to infect many others. These viruses are essentially so "hot" they burn themselves out. But vaccinated birds survive infection with the hot virus, and shed it for weeks—allowing strains that would otherwise die out to stick around, and kill any unprotected birds. The study appears in the journal PLoS Biology. [Andrew F. Read et al, Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens]

Of course, none of this is reason to doubt the efficacy of vaccines. "Vaccines have been one of the most important public health interventions. And the most cost effective we've ever had. And they're critically important in food chain security as well. So vaccines themselves are fantastic." But several imperfect vaccines for malaria are currently being tested. If they're approved, he says, we'll need to use other measures, like bed nets, to block transmission—remembering that not all vaccines are a one-shot deal.

—Christopher Intagliata

[The above text is a transcript of this podcast.]