<the laboratory-weakened virus used in the oral polio vaccine can very rapidly regain its strength if it starts spreading on its own. After a child is vaccinat

<the laboratory-weakened virus used in the oral polio vaccine can very rapidly regain its strength if it starts spreading on its own. After a child is vaccinated with live polio virus, the virus replicates inside the child's intestine and eventually is excreted. In places with poor sanitation, fecal matter can enter the drinking water supply and the virus is able to start spreading from person to person. "We discovered there's only a few [mutations] that have to happen and they happen rather quickly in the first month or two post-vaccination," Andino says. "As the virus starts circulating in the community, it acquires further mutations that make it basically indistinguishable from the wild-type virus. It's polio in terms of virulence and in terms of how the virus spreads." In June, the World Health Organization reported 15 cases of children paralyzed in Syria by vaccine-derived forms of polio. These cases come on top of two other vaccine-derived polio cases earlier this year in Syria and four in the Democratic Republic of the Congo.> https://www.npr.org/sections/goatsandsoda/2017/06/28/534403083/mutant-strains-of-polio-vaccine-now-cause-more-paralysis-than-wild-polio https://www.npr.org/sections/goatsandsoda/2017/06/28/5344030...