This study will be conducted in two stages. In the DTP booster phase subjects will receive a booster dose of Tritanrix™-HepB/Hib-MenAC or Tritanrix™-HepB/Hib (active control) at 15 to 24 months in a single-blind manner so that the subjects' parents will not know which vaccine was administered to their child (this booster phase is no longer recruiting). In the Mencevax™ ACWY phase at 24-30 months a dose of Mencevax™ ACWY will be given to subjects who were not boosted with a MenA conjugate and/or MenC containing vaccine at 15-24 months in an open manner (this booster phase is not yet recruiting). Up to four blood samples will be taken: before and one month after the administration of the DTP booster dose and of Mencevax™ ACWY. To comply with the immunisation calender of Thailand, at 15-24 months all subjects will receive OPV. At 16-25 months 2 doses of Japanese Encephalitis (JE) vaccine or a dose of varicella vaccine will be offered and at 25-31 months a dose of varicella or JE vaccine will be offered.