After receiving a birth dose of trivalent OPV , we enrolled and randomly assigned newborn babies ( 1:1:1:1 ) to receive two doses of mOPV1 with an interval of 1 week ( mOPV1-1 week ) , 2 weeks ( mOPV1-2 weeks ) , or 4 weeks ( mOPV1-4 weeks ) between doses , or two doses of bivalent OPV ( bOPV ) with an interval of 4 weeks between doses ( bOPV-4 weeks ) .