This study will compare ProQuad™ and concomitant administration of M-M-R™ II and Varivax™ with respect to immunogenicity, safety and tolerability. The primary hypothesis to be tested is that the antibody response rates to measles, mumps, rubella, and varicella 6 weeks after vaccination with ProQuad™ will be non-inferior to the antibody response rates after vaccination with concomitant M-M-R™ II and Varivax™.