Background: Basing early childhood immunization coverage rates on counts of recommended immunizations received by a target age provides limited information on how to address low rates in specific populations. Assessing immunization statuses at milestone ages, the points of time at which recommended immunizations are first late, is one alternative. One issue in milestone analysis is that immunization record data alone misclassifies many children with missed opportunities as having missed visits, as visits to providers on which no shots are delivered are not reported to immunization data systems
Objectives: To examine patterns of falling behind and catching up with recommended immunizations at milestone periods between birth and age two, and identify how missed opportunities and missed visits contribute to these changes.
Methods: Immunization histories for children born in 2005 and living in Oregon were assessed for immunization status at six milestone ages between birth and 24 months of age. Health care encounter data was matched to immunization data to determine whether missed visits or missed opportunities preceded under-immunization at each milestone.
Results: While 69% of children were up-to-date (UTD) with recommended immunizations by 24 months, only 32% of children were UTD at all milestone ages, and 9% were not UTD at any milestone. Missed opportunities accounted for a larger proportion of non-UTD children at each milestone age than missed visits. The percentage of non-UTD children with visits to providers on which no immunizations were given ranged from 21% for the 5 month milestone to 45% for the 24 month milestone.
Conclusions: Evaluating immunizations status at milestone ages allows for a clearer understanding of children’s progression through key immunization periods. Additionally, examining the contribution of missed visits and missed opportunities with a broader to UTD status, aids in identifying targeted intervention strategies