VDPVs that are not immunodeficiency-associated [116] are further categorized as (1) cVDPV when there is evidence of person-to-person transmission [56]; or (2) ambiguous VDPV (aVDPV): unique poliovirus isolates from a person with AFP with no known immunodeficiency or from an ES sample, and no evidence of transmission of genetically related virus [114,115,117,118]. This evidence concerns the gene AFP and immunodeficiency disease.