Reactions to a Second Dose of MMR Vaccine

Do rates of adverse events differ following the first and second doses of measles-mumps-rubella (MMR) vaccine? To find out, investigators compared rates of 13 parent-reported common adverse events occurring from 2 weeks before until 4 weeks after a first MMR dose (given to 535 infants ages 12 to 24 months) and a second dose (given to 633 children ages 4 to 6 years and 632 children ages 10 to 12 years). Concomitant administration of other vaccines was common among the infants (89%) and younger children (81%) but not among the older children (<1%).

Among the infants, rates of fever, diarrhea, and rash were significantly higher after the first dose than before the first dose; for example, fever incidence increased from 8% before the first dose …