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.

In addition, this study will evaluate if the effect of the age at first dose is modified by the type of vaccine administered.

Inclusion Criteria:

• Healthy male or female subject between 11 to 22 months at the time of first vaccination.
• Written informed consent obtained from the parent/guardian of the subject.

Exclusion Criteria:

• History of previous measles, mumps, rubella and/or varicella vaccination or disease or known exposure to any of these diseases within 30 days prior to the inclusion in the study.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, included the chronic administration (defined as more than 14 days) of immune-suppressants or other immune-modifying drugs within six months prior to the firs vaccine dose.
• History of any neurologic disorders or seizures.
• History of allergic diseases or reactions likely to be exacerbated by any component of the vaccines.
• Planned administration of a vaccine not foreseen by the study protocol from 30 days prior to each vaccination until 42-56 days after each vaccination

Subjects aged 11 to 22 months selected to participate in clinical trials to assess the safety and immunogenicity of the GSK biological Priorix-Tetra™ in Germany, Austria, Finland, Greece, Poland, Czech Republic, Italy, Lithuania, Norway, Romania, Russian Federation, Slovakia, Sweden, United States and Puerto Rico.