Trail To Evaluate the Immunity Duration of healthy children who already took part in " The safety and immunogenicity by different sequential schedules of bOPV and bOPV in dragee candy with sIPV, a randomized, double blind, single center and parallel phase Ⅲ clinic trial was performed in Infants of two-month old in Guangxi Province, China" and continue to search for the effects of booster immunization.

According to the requirement of the Strategy of Polio Eradication & Endgame Strategic Plan 2013-2018, bivalent oral attenuated live poliomyelitis vaccine against type 1 and 3 (bOPV) and inactivated poliomyelitis vaccine made by Sabin strain (sIPV) need to be used to eradiation both the wild poliovirus and vaccine-derived poliovirus. To evaluate the safety and immunogenicity by different sequential immunization schedules of bOPV and bOPV in dragee candy with sIPV,a randomized, double blind, single center and parallel phase Ⅲ clinic trial was performed in Guangxi Province in China. A total of 1200 infants at 2 months old were selected, and randomlydivided into 12 different groups (100 individuals were included in each group) administrated the vaccines at 0, 28, 56 days schedule.The detail of each group as following:1)1-dose cIPV + 2-dose bOPV (Candy); 2)1-dose sIPV + 2-dose bOPV (Candy); 3)2-dose cIPV + 1-dose bOPV (Candy); 4)2-dose sIPV + 1-dose bOPV (Candy); 5)2-dose cIPV + 1-dose tOPV (Candy); 6)2-dose sIPV + 1-dose tOPV (Candy); 7)1-dose cIPV + 2-dose bOPV (Liquid); 8)1-dose sIPV + 2-dose bOPV (Liquid); 9)2-dose cIPV + 1-dose bOPV (Liquid); 10)2-dose sIPV + 1-dose bOPV (Liquid); 11)2-dose cIPV + 1-dose tOPV (Liquid); 12)2-dose sIPV + 1-dose tOPV (Liquid).Blood Sample was collected before vaccination and one month after the third dose of vaccination. Neutralization antibody against type I, Type I and Type III poliomyelitis virus were detected to evaluate the seroprotection rates and antibody geometric mean concentrations. The fecal samples were collected to test viral shedding.The safety by different sequential schedule of the vaccines was also evaluated.This part of study have already been done in 2016.

To further evaluate the immunity duration of different sequential immunization schedules for bOPV and IPV ,more importantly,trying to research the effectiveness of bOPV booster immunization,the previous study will continue.

The detail of the research as following:

The subject, who have already receipted 3-dose polio vaccine for primary immunization in phase III clinical trail in Guangxi and with the result of the paired-serum, are recruited again.In order to research the immunity durability of primary immunizations of primary immunization(3 doses of immunization),blood samples are collected from the subject aged 24 months,36 months and 48 months separately.Neutralization antibody titers for against type I, Type I and Type III poliomyelitis virus are detected,moreover,the positive rate and antibody geometric mean titers are analyzed.

A single dose of bOPV（liquid/candy) will be given orally to subjects at the age of 48 months which can in support of the effectiveness research of booster immunization with bOPV. To protect the rights and interests of subjects,the investigator can use other poliovirus vaccine such as IPV instead when bOPV（liquid/candy）is not available.

The anticoagulant blood will be collected separatly at 48 months and the 28 days after booster immunization from subjects,which will be used to detect cellular immune response.

Inclusion Criteria:

• Subjects who have already taken part in phase 3 clinical trail in Guangxi and were vaccinated 3-dose primary immunization with polio vaccines .Moreover，the results of the selected paired serum are required.
• 24 months old(calendar month).
• Guardians understand the contents and requirements of this trail , meanwhile, voluntarily joined this study with informed consents.
• Able to attend all scheduled visits and to comply with all trial procedures(including vaccinate and blood collection)

Exclusion Criteria:

• Any booster immunization with polio vaccine after finishing 3-dose primary immunizations research.
• Polio virus infection was demonstrated in laboratory experiment.
• Participation in another clinical trial at the same times.
• Any condition that in the opinion of the investigator, may interfere with the evaluation of study objectives or increase the risk of subjects,such as acute or chronic diseases 、some abnormal detected by lab,and so on.