Sir.—As recommendations for the measles, mumps, and rubella (MMR) virus vaccine seesaw between 12 and 15 months of age, Faust and Thompson's article1 prompts the following suggestion: If a rigid lower limit of age must be set for MMR vaccine, let it be 11 months (335 days). Antibody titers change little for immunizations given at 11 months of age or thereafter. Wilkins and Wehrle2 found the following percentages of children with hemagglutination inhibition titers of 1:8 or greater, according to age at immunization: 9 months, 72%; 10 months, 84.6%; 11 months, 95.2%; 12 months, 95.4%; 13 months, 96%; 14 months, 95.2%; 15 months, 92.9%; 16 months, 100%; and 17 months and older, 93.2%.
Differences in measles attack rates, according to age at immunization, are negligible so long as the MMR vaccine is given at or after 11 months of age. Marks et al3 reported the only