Patterns of delayed immunizations among children in eight managed care settings across the United States

Background: Vaccine hesitancy is a growing public health problem, and many parents are requesting delayed immunization schedules for their children. The objective of this study was to describe individual vaccination patterns and trends over time in vaccine delay in a large cohort of children under two years of age. Methods: We used electronic administrative data to identify children born between 2004 and 2008 in eight managed care organizations participating in the Vaccine Safety Datalink. To quantify vaccine delay, we calculated the days undervaccinated for all vaccinations for children before age 2 years recommended by the Advisory Committee on Immunization Practices. We examined temporal trends and identified subtypes of vaccine delay. Results: Within a cohort of 324,633 children, we identified 176,524 (54.37%) who were undervaccinated for at least one vaccine before age 2 years. The annual percentage of undervaccinated children increased annually from 47.42% in 2004 to 62.40% in 2008 (p<.0001), and the average number of days undervaccinated increased from 182.13 (standard deviation [sd] = 173.31) to 200.19 (sd=187.51); p<.0001). Several patterns of alternative schedules were identified. For example, 1958 (<1%) children had not received any vaccines by 6 months of age, and 26,103 (8.04%) children did not receive Hepatitis B, measles-mumps-rubella, or varicella vaccines by age 2. Discussion: Within a large cohort of children under two years of age, overall trends indicate recent increases in children who may be intentionally unvaccinated or on alternative vaccination schedules. Effective risk communication strategies are needed to address this public health concern.