The Purpose of this study is to assess the seroconversion using inactivated poliovirus vaccine (IPV) and oral poliovirus vaccine (OPV) sequential schedule in pilot areas where IPV was phased introduced into national immunization program (NIP) in China.

Objective:

1. To evaluate the seroprotection rates after the 3 doses of primary polio vaccination among age eligible children in pilot areas where the IPV was introduced into NIP;
2. To evaluate the seroprotection rates after different dose(s) of polio vaccine(s) using sequential schedule.

Sample size:752

1. Jilin shall select 120 subjects to collect blood samples for evaluation of immune response,of whom 90 will be given OPV as second vaccine and 30 will be given IPV as second vaccine
2. Hubei shall select 132 subjects to collect blood samples for evaluation of immune response,of whom 102 will be given OPV as second vaccine and 30 will be given IPV as second vaccine
3. Ningxia shall select 260 subjects to collect blood samples for evaluation of immune response. The subject will be divided into 5 groups.
4. In Guangdong province,120 children will be selected as subjects , of whom 80 children will be given OPV as first vaccine and 40 children will be given IPV as first vaccine.
5. Tianjin shall select 120 subjects to collect blood samples for evaluation of immune response, of whom 80 will be given OPV as second vaccine and 30 will be given IPV as second vaccine

Organization Chart of the Study:

• Chinese Centers for Disease Control and Prevention (China CDC) lead the overall study, and be responsible for the study design, supervise the five selected provinces, conduct field monitor, data analysis and result distribution and explanation.
• The five pilot Provincial CDCs conduct the supervision and monitoring of 1-2 counties CDCs in each of pilot province.
• The County CDCs organize several vaccination clinics to recruit the subjects who meet the Inclusion criteria.

Quality Assurance Plan:

• China CDC and provincial CDC will monitor and supervise the field work, ensure the quality of the study.
• County CDCs organize vaccination clinics to service the subjects receiving timely polio vaccines in accordance with the immunization schedule, and that the blood samples are collected in accordance with the required timeline in difference groups.
• Clinic doctors will communicate and explain the purpose and procedure of the survey to parents, and the signed informed consent from by parents will be obtained before the children participate the study.
• The target drop-out rate of subjects without the second blood samples in each province should be less than 10%.
• The samples will be tested for polio antibody in a qualified laboratory, which should be a World Health Organization certified polio network laboratory in China.

Inclusion Criteria:

• Healthy children who are regular residents in the pilot counties where the immune response is evaluated in pilot provinces.
• Receiving the polio vaccination at 2, 3 and 4 months old with an interval of 28-35 days between doses according to national immunization schedule. The first dose of vaccine was administered at 2 full months old and less than 3 full months old.
• No rotavirus vaccine was administered during the period of primary polio vaccinations, and no vaccines of category 2 (private market) were administered simultaneously with polio vaccine. If vaccines of category 2 other than rotavirus vaccine were administered, they should be administered with a space of at least 2 weeks with polio vaccination.

Exclusion Criteria:

• The first dose of polio vaccine was administered at more than 3 months old regardless of the reasons.
• Children didn't get proper type and doses of vaccine as the protocal required.
• Children with known immunodeficiency conditions.
• Immunosuppressive agents or blood products were used after birth.
• Vaccine contraindications.

The participants will be seclected from the vaccination clinics. The study will be in five provinces, which conduct the pilot project of IPV phased introduction into national immunization program. In each province, 1-2 counties will be selected, and then selected several vaccination clinics.