In April 2017, WHO recommended that a two-dose, Inactivated Poliovirus Vaccine, IPV-only schedule at 4 months and 8 months of age can be used after polio eradication, with a schedule seroconversion target of at least 90%. However, there is no such data for China domestic Sabin strain IPVs to support a 2-dose schedule among Chinese infants.

This research is to determine the seroconversion rates among two arms:

Group 1: two-dose schedule, infants will be received two doses of Sabin IPV, the first dose at 4 month of age, and the second dose at 8-11 month of age.

Group 2: three-dose schedule, infants will be received three doses of Sabin IPV at 2, 3 and 4 month of age respectively. This schedule is currently recommended by manufacture's package insert for routine use.

The hypothesis is the seroconversion will be above 90% in both groups.

As a part of global polio eradication endgame immunization strategy, WHO requested all OPV-using countries to introduce at least one dose of IPV into the routine immunization schedule. Ideally IPV should be administered after 14 weeks of age, however an IPV-OPV sequential schedule is acceptable if VAPP is a concern. China stopped trivalent OPV in routine immunization in May 2016, introduced one dose of IPV at 2 months followed by three doses of type I / III bivalent OPV (bOPV) at 3 and 4 months and 4 years. A Sabin strain IPV was licensed in January 2015, and a second Sabin IPV was approved by China FDA in September 2017. The production capacity will increase in the next few years and will meet the demands of an IPV-only schedule in China. Other Sabin IPVs are in clinical trial stages, which will lead to sufficient IPV production for China to export IPV.

In April 2017, WHO/SAGE recommended that a two-dose, IPV-only schedule can be used after polio eradication, with a schedule seroconversion target of at least 90%. The recommendation was based in part on a study conducted in Cuba using Salk-IPV at 4 months and 8 months of age. Considering that risk of polio virus transmission is decreasing over time, and that IPV - either domestic Sabin-IPV or imported Salk-IPV - is expensive (6 USD/dose) for a program serving a birth cohort of 18 million, an efficient two-dose, IPV-only schedule will be very competitive with a more traditional 4-dose IPV-only schedule. However, for domestic Sabin IPVs, there are no such data to support a 2-dose schedule among Chinese infants.

The overall objective of this research is to determine the seroconversion rates of two reduced Sabin IPV schedules in Chinese children - a 2-dose schedule and a 3-dose schedule.

Group 1: two-dose schedule, infants will be received two doses of Sabin IPV, the first dose at 4 month of age, and the second dose at 8-11 month of age.

Group 2: three-dose schedule, infants will be received three doses of Sabin IPV at 2, 3 and 4 month of age respectively. This schedule is currently recommended by manufacture's package insert for routine use.

The hypothesis is the seroconversion will be above 90% in both groups.

Inclusion Criteria:

• Parent or legal guardian agree to participate in our study.
• The family is living locally with legal residency status.

Exclusion Criteria:

• Parent or legal guardian does not agree to participate in our study.
• The potential subject has received IPV or OPV before 4 months of age in the two-dose schedule group.
• The potential subject has one or more contraindications to IPV (although we will note the contraindications).