National trends in patterns of under-vaccination in early childhood: National Immunization Survey-Child, United States, 2011-2021.

Background: The study's objective was to examine national trends in patterns of under-vaccination in the United States.

Research design and methods: The National Immunization Survey-Child (NIS-Child) is an annual cross-sectional survey that collects provider-verified vaccination records from a large national probability sample of children. Records from the 2011-2021 NIS-Child were used to assess receipt of the combined 7-vaccine series by age 24 months. Based on prior work, patterns indicative of hesitancy included zero vaccines, not starting ≥1 series, and consistent vaccine-limiting. Patterns indicative of practical issues included starting all series but missing doses. Up-to-date (UTD) was defined as receiving all doses in the combined 7-vaccine series.

Results: The study population comprised 127,257 children. Over the observation period, patterns indicative of hesitancy significantly decreased (p-trend < 0.0001), patterns indicative of practical issues significantly decreased (p-trend < 0.0001), and UTD significantly increased (p-trend < 0.0001). In 2021, the weighted percentage in each category was as follows: probable hesitancy 6.3% (95% confidence interval [CI] 5.4%, 7.2%), probable practical issues 26.0% (95% CI 24.4%, 27.6%), and UTD 67.7% (95% CI 66.0%, 69.4%).

Conclusion: Over an 11-year period, vaccination coverage in the United States for the combined 7-vaccine series has improved, with patterns suggestive of practical issues or hesitancy declining.