The goal of this clinical trial is to explore two strategies to enhance nOPV2 immunogenicity in the field and overcome potential interference of bOPV:

1. Addition of IPV after one or several doses of nOPV2 and bOPV to close remaining immunity gaps;
2. Separation of bOPV and nOPV2 with an interval of 4 weeks.

Participants at 6 weeks of age will be enrolled and randomized to one of four arms receiving the different polio vaccines; nOPV2, bOPV and IPV, in different combination schedule. The target enrolment is 220 infants per arm for a total of 880. Blood will be collected from all participants to measure Poliovirus antibody titers to types 1, 2 and 3.

The study will assess whether adding IPV at the end of a 3-dose series of co-administered nOPV2+bOPV achieves at least a 95% seroconversion to all serotypes and whether adding IPV following two doses of co-administered nOPV2+bOPV achieves at least a 90% seroconversion to all poliovirus types.

Study findings will inform vaccination strategies in areas with co-circulation of poliovirus types and provide information about immunogenicity of potential routine immunization schedule with nOPV2 and bOPV in countries with persistent transmission or emergences of cVDPV2.

This is an open-label, controlled, inequality, four arm randomized clinical trial, will be conducted in two study sites in Dhaka, Bangladesh. Participants will be enrolled at 6 weeks of age following inclusion and exclusion criteria's, randomly assigned to one of the four study arms and followed to 18 weeks of age. Three polio vaccines will be used in the study: bOPV, nOPV2 and IPV in different sequential or combination schedules.

Entry evaluations will be completed at 6 weeks of age. Blood collection and study vaccine administration are planned during the study entry evaluation. Post-entry evaluations and follow-up will be done at 10 weeks, 14 weeks and 18 weeks of age. Blood will be collected before any study or EPI vaccines are administered. Presence of poliovirus neutralizing antibodies to all three poliovirus types will be assessed using a microneutralization assay.

Inclusion Criteria:

1. Healthy infants 6 weeks of age
2. Parents that consent for participation in the full length of the study.
3. Parents that can understand and comply with planned study procedures.

Exclusion Criteria:

1. Parents and infants are unable to participate in the full length of the study
2. A diagnosis or suspicion of immunodeficiency disorder either in the infant or in an immediate family member.
3. A diagnosis or suspicion of bleeding disorder
4. Acute diarrhoea, infection, or illness at the time of enrolment
5. Acute vomiting and intolerance to liquids within 24 hours before the enrolment visit
6. Evidence of a chronic medical condition
7. Receipt of any polio vaccine (OPV or IPV) before enrolment
8. Known allergy/sensitivity or reaction to polio vaccine, or its contents.
9. Infants from multiple births.
10. Infants from premature births (<37 weeks of gestation).

Study data will be jointly owned by icddr,b and CDC. After completion of study activities, summary tables of study results will be available on www.clinicaltrials.gov. After study registration, clinicaltrials.gov provides unrestricted public access on study details including summary tables.

Study investigators will have access to all participants' data without identifiers, including laboratory data. Co-investigators and technical advisors will have access to relevant data as defined in protocol

After study registration, clinicaltrials.gov. provides unrestricted public access on study details including summary tables.

After completion of study activities, summary tables of study results will be available on www.clinicaltrials.gov.

Study investigators will have access to all participants' data without identifiers, including laboratory data.

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