Recommendations for the timing of a second dose of measles-mumps-rubella vaccine (MMR2) differ. The Centers for Disease Control and Prevention Advisory Committee on Immunization Practices recommends that MMR2 be given between four and five years of age (before school entry, an age when access to the children by the physician is presumably greater). In contrast, the American Academy of Pediatrics recommends that the second dose be given between 11 and 12 years of age, when multiple guidance issues should be discussed and other vaccinations may be needed. Davis and associates analyzed data on clinical events following administration of MMR2 in these two age groups to compare any differences in the rate of adverse reactions.

Data were gathered from two health maintenance organizations (HMOs) that were members of the CDC Vaccine Safety Datalink study. MMR2 was routinely administered between four and six years of age at one HMO, and at the other, it was given between 10 and 12 years of age.

To be included in the analysis, children were required to have been enrolled in the HMO for at least three months before and three months after vaccination. Clinical events in the 30-day period after immunization were compared with those in a 30-day period before immunization to account for age-related differences in health care use. Visits to health care professionals within 30 days after vaccination were reviewed and information was collected on potential vaccine-related adverse events such as rash, fever, malaise, injection-site induration, seizures, neurologic and musculoskeletal symptoms, lymphadenopathy, thrombocytopenia, aseptic meningitis and joint pain.

MMR2 was received by 8,514 children between four and six years of age; 523 of these children were seen in the 30 days after immunization. Of these, 31 had symptoms potentially related to vaccination. A total of 18,036 patients received MMR2 between 10 and 12 years of age; 2,907 of these children were seen in the month after immunization. Sixty-eight of these children had symptoms potentially related to vaccination.

Compared with younger children, children who were 10 to 12 years old at the time of MMR2 administration were more likely to have health care visits for rash, seizures and joint pain (odds ratio: 1.78). The younger children had significantly fewer visits after the immunization (odds ratio: 0.52). Older girls were significantly more likely than older boys to develop joint pain and rash after immunization with MMR2 (odds ratio: 1.83 in girls versus 1.23 in boys).

The authors conclude that there are valid reasons for administering MMR2 between four and six years of age or between 10 and 12 years of age. The findings of their study suggest that the risk for adverse reactions is greater in children who receive MMR2 at an older age. As a limitation of their study, they note that it probably underestimated the true rate of clinical events following MMR2 immunization. Many children with mild symptoms following immunization do not seek medical care, and these children would have been missed in the authors' study. They also note that it is possible that the 10- to 12-year-old children may have been more likely than the younger children to seek medical care after MMR2 immunization.