'Leaky' Vaccines May Cause Super-Virulent Strains of a Virus to Survive

First Posted: Jul 28, 2015 08:32 AM EDT

Could vaccines cause more virulent viruses? Researchers have conducted experiments with the herpesvirus that causes Marek's disease in poultry and have found that some vaccines could allow more-virulent versions of a virus to survive.

"The challenge for the future is to identify other vaccines that also might allow more-virulent versions of a virus to survive and possibly to become even more harmful," said Andrew Read, one of the researchers, in a news release. "When a vaccine works perfectly, as do the childhood vaccines for smallpox, polio, mumps, rubella, and measles, it prevents vaccinated individuals from being sickened by the disease, and it also prevents them from transmitting the virus to others."

While "perfect" vaccines are a boon to the health industry, less-than-perfect vaccines could be a problem. In this case, the researchers took a closer look at Marek's disease, which has been on the rise in the poultry industry.

Marek's disease used to be a minor disease that didn't do much harm to chickens in the 1950s. Since then, though, the virulence of the virus has evolved and today it's capable of killing all unvaccinated birds in poultry flocks-sometimes within 10 days.

In this case, the researchers found out that the vaccine is a "leaky" vaccine. In other words, it allows super-virulent versions of the virus to survive. The researchers point out that this could be the case for avian influenza.

"In the United States and Europe, the birds that get avian influenza are culled, so no further evolution of the virus is possible," said Read. "But instead of controlling the disease by culling the infected birds, farmers in Southeast Asia use vaccines that leak-so evolution of the avian influenza virus toward greater virulence could happen."

The findings don't just have implications for chickens, though; they also have implications for human health. Researchers will need to be especially careful when creating next-generation vaccines to prevent the runaway evolution of more-virulent strains of viruses that can occur with leaky vaccines.

The findings are published in the journal PLOS Biology.

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TagsVirus, Vaccine, Influenza Virus, Vaccines, Chicken