Infant immunization and family size.

BACKGROUND To investigate whether a delay in infant immunization is associated with the number of older siblings. METHODS A cohort analysis of cumulative immunization uptake in 616 children aged 1-4 years recruited for a case-control study of atopic dermatitis in Norwich, UK was performed. The main outcome measures were the age of third pertussis and MMR immunizations. Delayed immunization was defined as a pertussis immunization age 6 months or greater, and MMR immunization aged 16 months or greater. RESULTS Having a larger number of older siblings was associated with a delay in pertussis immunization (6.2 per cent for children with no older siblings versus 23.3 per cent for children with two or more older siblings), but not in MMR immunization. CONCLUSION Infants with older siblings are at greater risk of pertussis infection from intrafamilial contagion yet are less likely to be immunized on time.