Allocation concealment: The allocation of each infant will be placed in sealed, opaque envelopes. For each infant presenting at six months of age, one envelope will be marked with the infant's name and then opened. At this stage, the sealed envelopes will contain one of two options viz (i) Vaccination at 6 months or (ii) No vaccination at 6 months. Those who receive the former will follow the protocol for Group A described below. Those who receive the latter will be followed-up at 7.5 months of age; at which stage a second set of sealed enveloped will be accessed. One envelope will be marked with the infant's name and then opened. At this stage, the sealed envelopes will contain one of two options viz (i) Vaccination at 7.5 months or (ii) Vaccination at 9 months. Those who receive the former will follow the protocol for Group B described below; and those who receive the latter will follow the protocol for Group C described below.