iVDPV= infective vaccine derived poliovirus
cVDPV= circulating vaccine derived poliovirus
aVDPV= ambiguous vaccine derived poliovirus
They are caused by a mutation regaining virulence in the attenuated poliovirus strains (Sabin 1-3) used for the oral poliomyelitis vaccine (OPV). Not sure of the specific difference between regressed strains though.