This is an immunogenicity study evaluating the development of the immune response of healthy infants following primary vaccination with Attenuvax at 6 or 9 months of age compared with responses in 12 month-old infants receiving MMR-II. Responses of infants receiving an early two dose measles vaccine regimen with the first dose given at 6 or 9 months followed by a second dose administered at 12 months will also be compared to infants given a single dose at 12 months of age (Table 2). The current approved regimen for measles vaccination is a first vaccination at 12-15 months and a subsequent vaccination at school entry. A secondary endpoint of this study will be to assess the safety of measles vaccine administered as Attenuvax at 6 or 9 months of age and in an early two dose measles vaccine regimen with Attenuvax administered at 6 or 9 months followed by MMR-II at 12 months of age.

This is an open-label immunogenicity and safety study of live attenuated Measles Vaccine (Attenuvax) and Measles-Mumps-Rubella (MMR-II) vaccine administered to healthy children at 6 (n=70), 9 (n=70), or 12 (n=70) months of age. Attenuvax (Measles Virus Vaccine Live, Merck \& Co., Inc) and MMR-II (Measles-Mumps-Rubella Virus Vaccine Live, Merck \& Co., Inc) will be delivered as 0.5 mL per dose, administered subcutaneously. Both vaccines are licensed for use in infants in the United States. MMR-II is recommended as part of the childhood immunization schedule by the AAP and ACIP, and Attenuvax is recommended for use as a single component vaccine in infants 6-11 months particularly for foreign travel and in measles outbreaks79. Infants vaccinated with Attenuvax at 6 or 9 months receive MMR-II at 12 months of age as recommended for follow-up vaccination. Immunization against measles is given as a trivalent formulation with mumps and rubella and is indicated as a primary dose at 12-15 months of age with a second dose recommended routinely at time of school entry, 4-6 years of age, but can be given at any earlier age provided that the interval between the first and second doses is at least 4 weeks . Two hundred and ten healthy children attending the Palo Alto Medical Clinic will be recruited into one of three cohorts over a five year period. (Table 1). All children in cohort 1 and 2 (6 and 9 month old infants) will receive Attenuvax as part of their participation in this study followed by MMR-II at 12 months of age as part of their routine WCC and not as a study vaccine. Infants recruited into cohort 3, (twelve month old infants) will receive one dose of MMR-II at 12 months. All infants in the study (cohort 1, 2 or 3) are receiving MMR-II at 12 months as part of their routine childhood vaccines. The vaccination schedules is summarized in Table 1. Other immunizations as required for routine WCC will be administered simultaneously. Participation will entail 2-4 visits, 2-3 blood samples, and 1-2 immunizations and will end after the 9-18 month visit, totaling 3 to 12 months time of participation for a subject depending on cohort and if participants opt for a second follow-up blood sample.