A vaccine schedule is the recommended organization on how, who, and when vaccines are delivered to recipients taking into account the age, risk of exposure, level and duration of initial immune response, and the prevailing service delivery mechanisms. An example of a schedule is the WHO recommended childhood schedule for most developing countries: BCG and OPV at birth; OPV1, DTP1 at six weeks; OPV2 & DTP2 at 10 weeks; OPV3 & DTP3 at 14 weeks and measles at 9 months.