, type 1 circulating vaccine-derived poliovirus (cVDPV) was isolated from one case patient with acute flaccid paralysis (AFP) and six unimmunized healthy contacts in isolated mountain villages in Guangxi, China. We conducted epidemiological investigations in the affected communities and nucleotide sequence analyses of the cVDPV isolates. The results of the investigations showed that the AFP patient, an unimmunized 10-year-old boy, and five laboratory-confirmed contacts lived in the same village; one contact lived in a neighboring village. Only ϳ27% of children 5 to 10 years of age in the affected villages had received three or more doses of the trivalent oral poliovirus vaccine (OPV). Nucleotide sequence analyses revealed that the cVDPV isolates differed from the Sabin 1 (S1) isolate at 1.4 to 2.2% of VP1 nucleotide positions and shared 12 nucleotide substitutions within VP1. All isolates were S1/S2/S1/S3 recombinants sharing common recombination junctions. Key determinants of attenuation were replaced. Phylogenetic analysis suggested that the cVDPV circulated locally for ϳ12 months following the initiating OPV dose. No VDPVs were found after mass OPV immunizations, conducted from May to June 2006, that targeted all children <12 years of age. Our findings reinforce the point that VDPVs can emerge and spread in isolated communities with immunity gaps. Maintenance of sensitive AFP and poliovirus surveillance is essential to permit early detection and a rapid response to VDPV circulation. China in 1994 (23, 38, 44). High levels of population immunity have been maintained throughout most of the country, and vigorous immunization responses to the detection of poliovirus circulation have subsequently protected against widespread poliovirus transmission. The WPVs (WPV type 1 [WPV1] and WPV3), introduced into communities bordering Myanmar in 1995 and 1996, were associated with only four paralytic poliomyelitis (polio) cases (44), and WPV1 imported into Qinghai, China, from northern India was associated with only one case in 1999 (45) Keys to the success in China are (i) a strong routine immunization program with trivalent oral poliovirus vaccine (tOPV) supplemented by synchronized mass campaigns in the form of national immunization days (NIDs) and subnational immunization days (SNIDs) (38), (ii) sensitive surveillance for cases of acute flaccid paralysis (AFP) (47), and (iii) rapid, detailed characterization of poliovirus isolates (23,47). The eradication of polio in China (40), whose population constitutes 23% of the world population, provided strong impetus to the World Health Organization's (WHO's) Global Polio Eradication Initiative, whose efforts have reduced the polio incidence by Ͼ99% since 1988, such that only four countries have never stopped WPV circulation (43).
Circulation of indigenous wild poliovirus (WPV) ceased inSuccessful eradication of WPV, however, does not entirely eliminate risks of paralytic poliomyelitis. The primary weapon in polio eradication, OPV, is genetically unstable and can revert to incr...