Comparison of vaccination with measles-mumps-rubella vaccine at 9, 12, and 15 months of age.

To determine seroconversion rates with measles-mumps-rubella vaccine administered to children at 9, 12, or 15 months of age, we undertook a prospective randomized trial. Among children vaccinated at 15 months of age, 98% seroconverted to measles, compared with 95% of those vaccinated at 12 months of age and 87% of those vaccinated at 9 months of age. In each age group, children of mothers born in or before 1963 had lower rates of seroconversion against measles, with the lowest rate in children vaccinated at 9 months. The seroconversion rate of rubella paralleled that of measles, with the lowest seroconversion rates in children vaccinated at 9 months of age whose mothers were born in or before 1963. The response to mumps varied little by age of the child or birth year of the child's mother. These results support the recommended age for first vaccination with measles-mumps-rubella at 12-15 months.