Immunogenicity nOPV2 With and Without bOPV

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).