Globally, polio cases have decreased by over 99% since 1988. However, wild poliovirus cases
continue in Pakistan. Conflict and lack of access to children due to on-going insecurity in
tribal areas present special challenges in interrupting transmission rapidly. There exists
limited knowledge on the effect of shorter intervals of mOPV vaccination on immunogenicity
levels in young children. The aim of this study is to demonstrate the non-inferiority of
shorter intervals (7 and 14 days) between doses of mOPV1 vaccine compared to the customary
interval (30 days). A 4th arm will receive bivalent (bOPV) vaccine at standard intervals
beginning at 6 weeks of age.