A new vaccination schedule with one dose of inactivated polio vaccine (IPV) followed by three doses of bivalent oral attenuated live polio vaccine (bOPV) was introduced in China in 2016. Both Sabin IPV (sIPV) and Salk IPV (wIPV) sequentially with bOPV were accepted in the Chinese routine vaccination schedule. We intended to assess the immunogenicity of the current primary schedule (s/wIPV-bOPV-bOPV) and the schedule in the early stage of the switch (tOPV-bOPV-bOPV), and compare immunogenicity between the groups with different polio virus strains. Healthy infants aged 60-89 days were recruited in hospitals in Chongqing. Infants were assigned to one of three treatments (tOPV-bOPV-bOPV, sIPV-bOPV-bOPV or wIPV-bOPV-bOPV) by enrollment time. Polio neutralizing antibody (NA) assays were conducted to assess immunity. 1027 eligible infants were enrolled. Over 95% seroprotection rates against type I poliovirus (PV1) and type III poliovirus (PV3) were observed in all groups. Infants who received tOPV-bOPV-bOPV had higher antibody titers against type II poliovirus (PV2) than did the IPV-bOPV-bOPV. The geometric mean titers (GMTs) of PV2 were only ~20 in the IPV-bOPV-bOPV. GMTs of PV1 were higher than PV3 in s/wIPV-bOPV-bOPV. The primary schedule of s/wIPV-bOPV-bOPV is insufficient to protect children against PV2, and the NA titer to PV3 is lower. Higher antibody responses were induced in sIPV-bOPV-bOPV than that in wIPV-bOPV-bOPV. Supplementary vaccination with one dose of IPV is necessary for children who had no tOPV immune history or had only one IPV to induce higher levels of immunity against PV2 and PV3.
Serums were collected from people to assess whether polio immunity level was high enough to satisfy the polio vaccine immunization switch in Chongqing. People in 7 age groups (<1 year, 1–2 years, 3–4 years, 5–6 years, 7–14 years, 15–19 years, ≧20 years) were randomly selected in 3 areas by different geographical features in 2015. Peripheral venous blood samples were obtained and assays to detect poliovirus (PV) –neutralizing antibodies were performed. Acute flaccid paralysis (AFP) data was collected from 2012 to 2016 in Chongqing to evaluate the performance of AFP surveillance system by indicator analysis. A total of 636 people were tested for PV neutralization antibodies (NA). Overall NA seroprevalence for PV1, PV2 and PV3 were 93.40%, 96.38% and 91.82%, and geometric mean titers (GMTs) were 61.14, 66.78 and 21.47, respectively. GMTs and NA seroprevalence for PV1, PV2 and PV3 in older people were lower than young people. There were significant differences in seroprevalences of PV1 and PV3 among geographic areas ( P < .05) in Chongqing. High seroprevalence for PV1, PV2, and PV3 and qualified capability for monitoring AFP cases showed that the polio eradication program has made positive achievements in Chongqing and established a stable base for a polio vaccine immunization switch. Nevertheless, GMTs were negatively associated with age in the geographic districts with poor economical features, which will increase the risk of emergence of vaccine-derived PV after polio vaccine switch. More than 1 dose of inactivated polio vaccine should be introduced into the polio vaccine schedule, and the supplementary immunization of polio should still be annually carried out after polio vaccine switch, especially among elder children and the adults.
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