Background To investigate whether a delay in infant immunization is associated with the number of older siblings. <br/>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. <br/>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. <br/>Conclusion Infants with older siblings are at greater risk of pertussis infection from intrafamilial contagion yet are less likely to be immunized on time
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