Immunogenicity of Different Primary Immunization Schedules with Inactivated Poliovirus Vaccine (IPV) Plus Pentavalent Vaccine (DTwP-HBV-Hib) or with Hexavalent Vaccine (DTwP-HBV-Hib-IPV)

The goal of this study is to provide information on immunogenicity at short and medium term for hexavalent with different schedules, which will be useful for the global polio program and countries, including Bangladesh. Primary objectives are 1. To compare the proportion of participants who seroconvert to all poliovirus serotypes four weeks after a primary immunization series. 2. To compare the proportion of participants seropositive against all poliovirus serotypes at 18 months of age. This is an open-label randomized clinical trial. Participants will be enrolled and randomized at 6 weeks of age to one of three arms. Target enrolment is 330 infants per arm and 200 controls; 990 in the main study and \~ 800 in the sub-study. A total of 4-5 blood samples will be collected from each infant before and after the primary vaccination series, and at 18 months of age, to assess systemic immune response to different antigens. Outcome measures/variables: Neutralizing antibody titers in serum will be quantified for poliovirus types 1, 2, and 3 using a microneutralization test; for diphtheria toxoid, tetanus toxoid, and pertussis toxin using a Multiplex bead assay; and for antibodies to hepatitis B surface antigen (anti-HBs) using serologic assay. The presence of poliovirus types 1 and 3 in oropharyngeal swabs and stools following the bOPV challenge will be tested using a real-time reverse transcription PCR (rRT-PCR) assay.