 Oral vaccines can also give rise to persistent infection in immunodeficient individuals, with the virus eventually mutating into a more virulent immunodeficiency-associated vaccine-derived poliovirus (iVDPV).In particular the type2 strain seems prone to reversions, so in 2016 the eradication effort abandoned the trivalent oral vaccine containing attenuated strains of all three virus types, and replaced it with a bivalent oral vaccine lacking the type2 virus.Eradication efforts will eventually require all oral vaccination to be discontinued in favor of the use of injectable vaccines.