Immunogenicity and Safety Study of Proquad® and Infanrix® Hexa When Administered Concomitantly (V221-035)

Primary Objective: * To demonstrate that ProQuad® can be administered concomitantly with a booster dose of Infanrix® hexa to healthy children 12 to 23 months of age without impairing either the antibody response rates to measles, mumps, rubella, varicella, hepatitis B and Haemophilus influenzae type b; or to the 3 pertussis antibody titres measured at 42 days following vaccination. Secondary Objectives: * To describe the antibody titres and the antibody response rates to measles, mumps, rubella, varicella, diphtheria, tetanus, pertussis, hepatitis B, poliomyelitis and Haemophilus influenzae type b as measured at 42 days following vaccination by an Infanrix® hexa primary series schedule and all data are pooled. * To evaluate the safety profile of ProQuad® when administered concomitantly with a booster dose of Infanrix® hexa by an Infanrix® hexa primary series schedule and all data are pooled.