KEYWORDS:
immunization status, vaccination coverage, late start

BACKGROUND:
Children who start their immunization late are at risk for falling behind. The patterns and dynamics of these relationships are not well understood.

OBJECTIVE(S):
To describe the relationship between age at first immunization and subsequent immunization status.

METHOD(S):
In April 1999, we audited the records of 768 randomly selected 18-35 month-olds at 11 practices in a low-income community in New York City. Supplemented immunization data was obtained from New York Citywide Immunization Registry records. We analyzed the distribution of age at first DTaP in two-week intervals relative to 4:3:1:3 up-to-date (UTD) at 12 and 18 months, per ACIP.

RESULT(S):
22% of children received DTaP1 before 8 weeks of age, 48% between 8-9 weeks, 17% between 10-12 weeks, and 13% at 13+ weeks. UTD at 12 months progressively declined with delay at receipt of DTaP1, from 85% for children receiving DTaP1 before 8 weeks to 63% for those initiating vaccination at 13+ weeks. At 18 months, the difference in UTD narrowed from 13% to 9%. Compared to children immunized before 10 weeks, children with DTaP1 at 10-12 weeks were 1.6 times less likely to be UTD at 12 months, while those first immunized at 13+ weeks were 3 times less likely to be UTD at 12 months. A similar pattern was observed at 18 months, but it was only statistically significant for children immunized before versus after 10 weeks of age.

CONCLUSIONS(S):
Children who start their immunizations on time are more likely to stay current with their immunizations throughout the first year of life. The effect persists into the second year of life, but is moderated as the catch-up period lengthens.

LEARNING OBJECTIVES:
To emphasize the importance of early immunizations.