Immunization status and birth order
S. J. Schaffer and P. G. Szilagyi
Department of Pediatrics, University of Rochester (NY) School of Medicine and Dentistry, USA.
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.
