The purpose of this booster vaccination study is to evaluate the persistence and immune memory induced by a three-dose primary vaccination course with GSK Biologicals' MenC-TT (Neisseria meningitidis group C polysaccharide-tetanus toxoid) and GSK Biologicals' Hib-MenC-TT (Haemophilus influenzae type b-MenC-TT) or Meningitec™ vaccines in healthy toddlers aged 12-15 months by giving them a 1/5th dose of Mencevax™ ACWY.