 The oral vaccine is a powerful tool in fighting polio in part because of its person-to-person transmission and resulting contact immunity.However, under conditions of long-term circulation in under-vaccinated populations, the virus can accumulate mutations that reverse the attenuation and result in vaccine virus strains that themselves cause polio.As a result of such circulating vaccine-derived poliovirus (cVDPV) strains, polio outbreaks have periodically recurred in regions that have long been free of the wild virus, but where vaccination rates have fallen.