This is a continuous study of preliminary trial (Immunogenicity of three sequential schedules with Sabin inactivated poliovirus vaccine and bivalent oral poliovirus vaccine in Zhejiang, China: NCT03147560) . The investigators will follow up previous participants at age 4 years old with different primary sequential immunization schedules at age 2, 3 and 4 months: sIPV-bOPV-bOPV , sIPV-sIPV-bOPV , or sIPV-sIPV-sIPV, and give the 4th doses of polio vaccine with Sabin IPV or bOPV. A pre-vaccination blood sample is taken for polio antibody determinations. Sabin IPV will be administrated for participants in the previous Group 1 (sIPV-bOPV-bOPV), and Sabin IPV or bOPV will be given for participants in the previous Group 2 (sIPV-sIPV-bOPV) and Group 3 (sIPV-sIPV-sIPV) after they were randomly divided into two subgroups. At least two active surveillance visits, during which staff visited the participants s at home (on days 3 and 30) will required after vaccination to collect adverse reaction data. Parents or legal guardians were instructed to contact investigators if they observed any possible vaccine-related adverse events or important medical events (such as serious illness, physical disability, or death). The second blood sample will be collected 28-60 days after the booster vaccination. The investigators will asess the immunogenicity by seroconversion rate and seropositivity after the booster vaccination, and evaluate the antibody persistence using seropositive rate and GMT in participants before the booster vaccination. Safety will assessed with the occurrence of adverse events reported across the study groups.
This study is to evaluate the immunogenicity and safety of Sabin IPV or bOPV, given as a booster vaccination in children aged 4 years who were previously immunised with different sequential immunization history by Sabin IPV and bOPV, and to observe the antibody persistence three years after different primary sequential immunization with Sabin IPV or bOPV at age 2, 3 and 4 months.