The study is open. However, the 4 different formulations of GSK's MenACWY conjugate vaccine will be administered in a double-blind manner. Mencevax™ ACWY vaccine will serve as active control. Subjects will receive one vaccine dose only (GSK's MenACWY conjugate vaccine or Mencevax™ ACWY vaccine), and will have 2 blood samples taken, before and one month after vaccination.