A parent asks when their child should receive the first MMR (measles, mumps, rubella) vaccine. What is the correct response?

MMR VACCINE SCHEDULE (CDC ACIP recommended): FIRST DOSE: 12-15 months of age; SECOND DOSE: 4-6 years of age (school entry); CATCH-UP: Children who did not receive MMR on schedule should be vaccinated as soon as possible; adults born after 1957 who lack evidence of immunity should receive 1-2 doses. WHY NOT BEFORE 12 MONTHS: Maternal antibodies (IgG passed to the baby across the placenta) interfere with the baby's immune response to live vaccines; these antibodies decline over the first year; by 12 months, most infants have sufficiently low maternal antibody levels for the vaccine to generate an effective immune response; some outbreak situations (international travel, exposure risk) may prompt vaccination at 6-11 months — but this early dose does NOT count toward the series and must be repeated at 12-15 months. MMR PRECAUTIONS (live vaccine): Do NOT administer to immunocompromised individuals (HIV with low CD4 count, cancer treatment, corticosteroid use); do NOT administer during pregnancy (teratogenic risk); avoid pregnancy for 4 weeks after vaccination; may cause fever and mild rash 7-12 days post-injection (expected immune response); egg allergy is not a contraindication unless severe anaphylaxis to egg — consult allergist; TST (tuberculin test) if needed should be done on the same day as MMR or 4-6 weeks later (MMR can suppress tuberculin reaction temporarily).