http://medicalxpress.com/news/2015-04-emerging-poliovirus-threatens-eradication.html

Circulating vaccine-derived polioviruses are "the biggest surprise" that scientists have encountered in their work to end polio, said Walter Orenstein, MD, lead author of a 2014 clinical report about polio eradication from the American Academy of Pediatrics and a professor of medicine at Emory University. The viruses "are genotypically vaccine, but phenotypically wild virus," Orenstein explained; in other words, they look genetically like the vaccine, but behave in nature like their wild counterparts.

How a mutated virus regains power

These vaccine-derived viruses are escapees. They start in the intestines of a tiny fraction of people who receive the oral polio vaccine. The oral vaccine uses live, but attenuated, strains of poliovirus, meaning the pathogen has been altered to be harmless—or at least much less virulent. In about one in a million people, though, the attenuated poliovirus mutates in such a way that it regains its potency. Sometimes, it sickens the individual who got the vaccine, a problem known to doctors for decades.

What's come as a surprise, however, is that even if these mutated viruses fail to affect the person in whom they originate, they may escape into the environment in the individuals' feces and start infecting people like wild viruses do: In areas with poor sanitation systems, untreated sewage spreads the new viruses to others, who can become sickened. For instance, in Nigeria during the first nine months of 2014, 21 polio cases were caused by the circulating vaccine-derived viruses, compared to six cases from the wild virus, according to a report from the U.S. Centers for Disease Control and Prevention. In the same period, Pakistan had 22 cases from circulating vaccine-derived viruses and 170 from wild viruses, while Afghanistan had no cases from vaccine-derived viruses and nine from wild poliovirus, the CDC reported.

The risk is prompting public health experts to re-examine how best to protect children in developing countries from polio. For example, with a grant from the Bill & Melinda Gates Foundation, Maldonado is collecting data in communities in Mexico that are changing their polio-vaccination strategy.

"It's important to understand how we can phase out the live-virus vaccine, so that we can use the killed vaccine in the best way possible," Maldonado said, referring to a vaccination method that uses an injection of dead poliovirus. "We want not only to prevent circulation of natural polio, but also to understand how to prevent the viruses derived from the live vaccine from becoming the next source of paralysis in communities."

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