Immunization status and birth order.

OBJECTIVE

To determine whether an association exists between immunization status and birth order.

DESIGN

Medical record review of immunization dates for matched siblings.

SETTING

Pediatric clinic at a university medical center.

SUBJECTS

A total of 892 children (446 sibling pairs of firstborn and secondborn children) born between 1983 and 1991 who received regular pediatric care at the clinic.

OUTCOME MEASURES

Median ages at which firstborn children and their secondborn siblings had been immunized with the initial four doses of diphtheria and tetanus toxoids and pertussis vaccine (DTP1, DTP2, DTP3, and DPT4) and the initial dose of measles-mumps-rubella vaccine; point prevalences of firstborn and secondborn children up-to-date with all immunizations at each 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 firstborn children (P values ranging from < .0001 to < .05). Firstborn children were much more likely than their secondborn siblings to have been immunized 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 their 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 delayed immunizations.