Immunogenicity and Safety Study of GlaxoSmithKline (GSK) Biologicals' Combined Measles-mumps-rubella (MMR) Vaccine in Subjects Four to Six Years of Age

The GSK Biologicals' MMR vaccine (Priorix®) and Merck's MMR vaccine (M-M-R®II) are referred to as Inv\_MMR vaccine and Com\_MMR vaccine respectively. 2 lots of the comparator vaccine (Com\_MMR\_L1 and Com\_MMR\_L2) will be used, but the 2 lots will be analysed as a pool. The Inv\_MMR vaccine will be administered as a second dose to children who already received a first dose Com\_MMR vaccine. Since the second dose of a MMR vaccine in the US is routinely co-administered with DTaP-IPV vaccine (Kinrix®) and varicella vaccine (VV) (ProQuad® or Varivax®), some children will receive one dose of these vaccines along with either of the MMR vaccines.