“…This definition follows from the high rate of nucleotide sequence evolution in poliovirus (ϳ1% per year) (9) and the normal period of poliovirus excretion of Ͻ3 months (10). VDPVs are distributed into three categories: (i) circulating VDPVs (cVDPVs) associated with outbreaks in settings of low OPV coverage (8,11,12), (ii) immunodeficiency-associated VDPVs (iVDPVs) from prolonged infections of persons with primary immunodeficiencies (8,11,(13)(14)(15)(16), and (iii) ambiguous VDPVs (aVDPVs) isolated from immunocompetent persons or the environment (8,11,12). Environmental aVDPVs in countries with high poliovirus vaccine coverage may signal latent chronic infections, but none of the infected persons have been identified so far (3,8,(17)(18)(19)(20).…”