“Leaky” Vaccines May Improve Virus Transmission, Study Finds

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A new study on poultry finds that imperfect vaccines can result in more persistent deadly diseases.

The findings revolve around a concept called the “imperfect vaccine hypothesis”. The imperfect vaccine hypothesis suggests that “leaky” vaccines—those that protect the host but do not prevent contagion—can actually prolong the life of infections which normally kill their host before they can reproduce. This allows the disease time to circulate within the host and spread to others. In essence, the leaky vaccine creates a Typhoid Mary situation where the inoculated person is healthy but remains capable of spreading the disease.

The experiments measured “viral shedding”, the amount of a virus that is released from a host after they are infected. During the experiments, chickens were divided into vaccinated and non-vaccinated groups, infected with Marek’s disease (a poultry-borne illness) of varying strengths, and the amount of viral shedding was monitored. The researchers found that chickens given imperfect vaccines actually prolonged the life of the most deadly strains of Marek’s disease and resulted in heightened viral shedding into the environment. In other words, the strain normally too lethal to persist in a non-vaccinated population was allowed to remain and spread thanks to the vaccine.

It was also theorized that the leaky vaccines (which could give diseases more time to mutate) were one of the reasons why, after the 1950’s, Marek’s disease saw an increase in the number of highly lethal strains. This relationship has some statistical correlations, but the researchers emphasized that the evidence of the vaccine’s role is not as clear as, say, the development of antibiotic-resistant germs.

The researchers noted that none of the viral behaviors linked to leaky vaccines have ever occurred in humans. They do hope that their findings encourage vaccine developers to more closely consider how well their vaccines prevent viral shedding, and urge more caution when implementing animal vaccines.

Sources for Today’s Article:
“‘Leaky’ Vaccines May Strengthen Viruses: Study.” CTVNews web site, July 29, 2015, http://www.ctvnews.ca/health/leaky-vaccines-may-strengthen-viruses-study-1.2492523.
Read, A.F., et al., “Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens,” PLoS Biology PLoS Biol, 2015; last accessed July 29, 2015. doi:10.1371/journal.pbio.1002198.

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