In 2017, China CDC conducted a study titled "A Study of Immunogenicity and Seroconversion With Sabin IPV Schedules in China". At present, all the infants in the monitoring cohort were over 18 months old and at least one year had elapsed since the last dose of sIPV inoculation.Since there is no any data about the protection duration of two-dose Sabin IPV schedules, China CDC propose to follow up the study cohort to evaluate the proportion of seroprotection of antibody and measure neutralizing antibody titers against poliovirus at over 18 months of age infants after a 2-dose or a 3-dose primary schedule of Sabin IPV in Chinese children.

In 2017, China CDC conducted a study titled "A Study of Immunogenicity and Seroconversion With Sabin IPV Schedules in China". This study offers evidence that two doses of sIPV administered at 4 and 8-11 months of age and three doses of sIPV administered at 2, 3, and 4 months of age both provide serological protection against poliomyelitis. At present, all the infants in the monitoring cohort were over 18 months old and at least one year had elapsed since the last dose of sIPV inoculation.

Since there is no any data about the protection duration of two-dose Sabin IPV schedules, China CDC propose to follow up the study cohort when all subjects are at least 18 months old to monitor persistence of antibody. So that, if the antibody levels are not high enough to protect children from the disease, remedial measures can be taken as soon as possible.So China CDC conduct this follow up study ,to

1. Evaluate the proportion of seroprotection of antibody against poliovirus at over 18 months of age infants after a 2-dose or a 3-dose primary schedule of Sabin IPV in Chinese children.
2. Measure neutralizing antibody titers against poliovirus type I, II and III at over 18 months of age infants after 2-dose Sabin IPV schedule, compared with 3-dose Sabin IPV schedule.

Inclusion Criteria:

• Parent or legal guardian agree to participate in the follow study.
• The subject was vaccinated either two doses or three doses Sabin IPV ,collected two blood samples and laboratory test results were available per protocol of pervious study
• The subject is at least 18 months age.

Exclusion Criteria:

• Parent or legal guardian does not agree to participate in the follow up study.
• The subject has received any polio vaccine product beyond the protocol of previous study.

Children who had participated the previous Sabin IPV study in 6 counties in Shandong province.