How does the oral polio vaccine help stop outbreaks in communities?

The oral polio vaccine contains a live attenuated poliovirus that trains the immune system in the intestine, where poliovirus first grows after infection. Because it is taken by mouth, it is easy to deliver in mass campaigns. In some settings it helped interrupt transmission because vaccinated people could shed weakened virus that spread immunity indirectly. However, because it is live, rare vaccine-derived poliovirus outbreaks can occur, which is why many countries now also use the inactivated polio vaccine.