This is an open-label randomized clinical trial that will compare immune responses among infants who receive either novel monovalent oral poliovirus vaccine type 2 (nOPV2) alone, bivalent oral poliovirus vaccine (bOPV) alone, or co-administered nOPV2 and bOPV.

It is expected that nOPV2 would replace mOPV2 for responding to type 2 outbreaks. Outbreak response for cVDPV2 also offers the opportunity to close immunity gaps to polioviruses types 1 and 3. Furthermore, GPEI might have to respond to two poliovirus outbreaks in the same geography. For either scenario, it would be important to get data on the immunogenicity of co-administered nOPV2 and bOPV, compared to either vaccine given alone.

This clinical trial assesses and compares the immunogenicity of nOPV2 given with or without bOPV. Healthy infants 6 weeks of age will be enrolled in Dhaka, Bangladesh, and randomized to one of three study arms â " Arm A: nOPV2 only, Arm B: nOPV2 and bOPV, or Arm C: bOPV only. Infants will be followed-up until 18 weeks of age through clinic and household visits. Blood specimens will be collected to test for immunological response. Stool specimens will be collected from infant vaccine recipients and one sibling household contact each to assess viral recombinants and nOPV2 household transmission.

Inclusion Criteria:

• Healthy infants 6 weeks of age (range: 42-48 days).
• Parents that consent for participation in the full length of the study.
• Parents that can understand and comply with planned study procedures.
• Infant has at least one sibling aged <10 years living in the same household that is eligible for participation in the study.

Exclusion Criteria:

• Parents and infants who are unable to participate in the full length of the study (e.g., plan to move away from the study area during the study period).
• A diagnosis or suspicion of immunodeficiency disorder either in the infant or in an immediate family member.
• A diagnosis or suspicion of bleeding disorder that would contraindicate administration of bOPV or nOPV2 or collection of blood by venipuncture.
• Acute diarrhoea, infection or illness at the time of enrolment (6 weeks of age) that would require infant's admission to a hospital.
• Acute vomiting and intolerance to liquids within 24 hours before the enrolment visit (6 weeks of age).
• Evidence of a chronic medical condition identified by a study medical officer during physical exam.
• Receipt of any polio vaccine (OPV or IPV) before enrolment based upon documentation or parental recall.
• Known allergy/sensitivity or reaction to polio vaccine, or its contents.
• Infants from multiple births. Infants from multiple births will be excluded because the infant(s) who is/are not enrolled would likely receive OPV through routine immunization and transmit vaccine poliovirus to the enrolled infant. Even if all births from a multiple birth could be enrolled in the study, we will exclude multiple births as discontinuation of one may lead to discontinuation of multiple participants.
• Infants from premature births (<37 weeks of gestation).

Aggregated data will be added to the registration with publication. In accordance with the protocol, icddr,b investigators will have access to participant data with identifiers. External investigators will have access to deidentified participant data. Deidentified data may be shared with national and international vaccine manufacturers and regulatory authorities upon request.