RecruitingPhase 3Biological interventionA Study on the Immune Response and Safety of an Investigational Chickenpox Vaccine and a Marketed Measles, Mumps and Rubella Vaccine When Administered as Intramuscular Injection to Healthy Children 12 to 15 Months of AgeThis study aims to assess the immune response and safety of GSK's candidate chickenpox and marketed MMR vaccines when given to children 12 to 15 months of age via a muscle injection. It compares the GSK vaccines to Merck's chickenpox vaccine, administered just under the skin. Additionally, the study will evaluate the immune response and safety of giving the GSK vaccines along with other childhood vaccines through a muscle injection.