'Leaky' Vaccines May Create Stronger Viruses

A worker vaccinates a 4-week-old chick at a poultry farm. Athar Hussain/REUTERS

An international team of scientists have confirmed that vaccines can contribute to the development of virulent virus strains.

The controversial "supervirus" theory, known widely from the 2011 Steven Soderbergh film Contagion, purports that vaccinations, when they fail to completely eliminate a disease in an individual or a population, can cause viruses to evolve new traits that make them more resistant to treatment. The new study, published in PLOS Biology, refers to these imperfect vaccines as "leaky vaccines." Vaccines for diseases like smallpox and measles are considered "perfect" because they create lifelong immunity and prevent diseases from being spread. "Leaky" vaccines, on the other hand, which are used for diseases like malaria and avian flu and commonly given to livestock, immunize the individual but do not prevent viruses from spreading. In some cases, a disease can still spread even after the host has died.

A team of researchers led by Andrew Read of Penn State intentionally administered "leaky vaccines" for Marek's disease, a strain of herpes that induces paralysis in poultry, to groups of chickens, and discovered that unvaccinated individuals coming into contact with the vaccinated subjects had been infected with a "hot" viral strain.

In a Monday press release, Venugopal Nair, a professor at the The Pirbright Institute in the U.K. and one of the principal authors of the study, said, "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."

Read says that it is critical to ensure that "next generation vaccines" for diseases like Ebola are not "leaky," lest deadlier versions of diseases spread to the unvaccinated. In their experiments, the researchers found that unvaccinated individuals are at the greatest risk. The study does not suggest that someone who has been vaccinated is likely to contract a supervirus, and the researchers advocate for more widespread vaccination.

"The future challenge is to identify whether there are other types of vaccines used in animals and humans that might also generate these evolutionary risks," the study says. Easily spread viruses with long infectious periods are more likely to develop into superviruses than less contagious strains, according to the study.