IMMUNIZATION STATUS AND BIRTH-ORDER

Objective: To determine whether an association exists between immuniza tion status and birth order. Design: Medical record review of immuniza tion dates for matched siblings. Setting: Pediatric clinic at a univer sity medical center. Subjects: A total of 892 children (446 sibling pa irs of firstborn and secondborn children) born between 1983 and 1991 w ho received regular pediatric care at the clinic. Outcome Measures: Me dian ages at which firstborn children and their secondborn siblings ha d been immunized with the initial four doses of diphtheria and tetanus toxoids and pertussis vaccine (DTP1, DTP2, DTP3, and DPT4) and the in itial dose of measles-mumps-rubella vaccine; point prevalences of firs tborn and secondborn children up-to-date with all immunizations at eac h month of life to 2 years of age. Results: Between 5 and 12 months of life, the percentage of secondborn children who were fully immunized was significantly lower than the percentage of fully immunized firstbo rn children (P values ranging from <.0001 to <.05). Firstborn children were much more likely than their secondborn siblings to have been imm unized on time with DTP2 (z=3.80, P=.0001) and DTP3 (z=3.31, P=.0009). Overall, DTP2 immunizations were given at median ages 10 days later, and DTP3 immunizations, 20 days later to secondborn children than to t heir firstborn siblings. In addition, late immunization of a firstborn child was found to increase the risk that a secondborn sibling would also be immunized late. Conclusions: Secondborn children are likely to be immunized later than firstborn children. Secondborn children with an older sibling who was immunized late are at particular risk for del ayed immunizations.