When and why children fall behind with vaccinations: missed visits and missed opportunities at milestone ages.

OBJECTIVE: Little is known about when-and why-children fall behind in their recommended vaccinations. Vaccination status throughout the first 2 years of life was examined to identify vulnerable transition periods that account for attrition and to determine whether children fell behind because they missed vaccination visits or because of missed opportunities for simultaneous vaccination.
METHODS: Vaccination histories for 27,083 children aged 24-35 months in the 2006-2007 National Immunization Survey were analyzed to determine the vaccination status at each age in days, focusing on the milestone ages of 3, 5, 7, 16, 19, and 24 months. Also assessed were the percentage of children who fell behind between milestones and the percentage who did so due to the lack of a vaccination visit compared to a missed opportunity for simultaneous vaccination.
RESULTS: The percentage of children who fell behind from one milestone age to the next ranged from 9% during the interval from age 16 months to 19 months to 20% during the interval from age 7 months to age 16 months. Missed vaccination visits accounted for most attrition during the intervals from age 3 months to age 5 months, age 5 months to age 7 months, and age 16 months to age 19 months, while missed opportunities for simultaneous vaccination accounted for >90% of the children who fell behind during the interval from age 7 months to age 16 months.
CONCLUSIONS: Missed vaccination visits and missed opportunities for simultaneous vaccinations both must be addressed to reduce the number of children falling behind in their vaccinations. With one in five children falling behind during the interval from age 7 months to age 16 months--mostly as a result of missed opportunities for simultaneous vaccination--providers should focus on this time interval to deliver all of the recommended vaccinations that are due.