This study will evaluate the consistency of the immune response to three different lots of GSK Biologicals' trivalent investigational MMR vaccine (referred to as Inv_MMR vaccine, throughout this document) and compare its immunogenicity to the US standard of care comparator vaccine (M M R®II, Merck and Co., Inc., referred to as Com_MMR throughout this document) in children during their second year of life. The Inv_MMR vaccine will be given as one of three consistency lots manufactured to target potencies designated as Inv_MMR_L1, Inv_MMR_L2 and Inv_MMR_L3. The Com_MMR vaccine will be given as one of two lots designated Com_MMR_L1 and Com_MMR_L2 and will be analyzed as pooled lots within the study. The MMR vaccine will be co-administered with Varivax, Havrix and (in the US sub-cohort only) Prevnar 13 which are routinely administered to children of this age in the US.