Healthy children who received two doses of GlaxoSmithKline (GSK) Biologicals' live attenuated measles-mumps-rubella vaccine (MMR) or measles-mumps-rubella-varicella vaccine (MMRV) in their second year of life.The purpose of this study is to assess the effect of the age at administration of the first dose on the reactogenicity and immunogenicity of the measles component of these vaccines.