 Likewise, mutation during the course of persistent circulation in undervaccinated populations can lead to vaccine-derived poliovirus strains (cVDPV) that can induce polio at much higher rates.Until recently, a trivalent OPV containing all three virus strains was used, but with the eradication of wild poliovirus type2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, while use of monovalent type2 OPV is restricted to regions with documented cVDPV2 circulation.The inactivated polio vaccine (IPV, or Salk) contains trivalent fully inactivated virus, administered by injection.