Background: The CDC conducts the National Immunization Survey-Child (NIS-Child) annually to monitor U.S. vaccination coverage. As compared with coverage metrics, timeliness metrics, such as days undervaccinated, provide a more sensitive measure of whether children received vaccines within age intervals recommended by the U.S. Advisory Committee on Immunization Practices (ACIP). The 2021 NIS-Child included children who received vaccines before and during the COVID-19 pandemic.
Objectives: To compare timeliness of combined 7-vaccine series receipt (≥4 diphtheria-tetanus-acellular pertussis, ≥3 poliovirus, ≥1 measles-mumps-rubella, ≥3 or ≥4 Haemophilus influenzae type b, ≥3 Hepatitis B, ≥1 varicella, and ≥4 pneumococcal vaccination doses) among U.S. children ages 0-19 months in the 2019 and 2021 NIS-Child.
Methods: The NIS-Child public files include provider-verified age in days, but not dates, that surveyed children received vaccines. We limited our analyses to children ages 19-23 months at the time of survey. As of the start of March 2020 lockdowns, study-eligible children in the 2019 NIS-Child were over 22 months-old, and study-eligible children in the 2021 NIS-Child were 0-13 months-old. We used linear regression to compare days undervaccinated, which compares when a child received vaccinations relative to ACIP recommendations (allowing for a 30-day grace period) by age 19 months. Using a multivariable log-linked regression model, we identified whether survey year, socioeconomic or demographic factors were associated with on-time vaccination. Analyses accounted for the complex survey design.
Results: Across the combined 7-vaccine series, average days undervaccinated was 64.3 days (SE=3.75) among n=5,068 children in the 2019 NIS-Child and 58.9 days (SE=3.02) among n=5,048 children in the 2021 survey (p=0.26). The percent of children who received all vaccine doses on-time was 33.4% (95% CI: 30.6-36.1) in the 2019 cohort and 34.8% (95% CI: 32.3-37.3) in 2021 cohort (Rao-Scott chi-square p-value=0.12). Survey year was not associated with on-time vaccine receipt (adjusted prevalence ratio [aPR]=1.00, 95% CI=0.90-1.11). Having Medicaid insurance (aPR=0.72, 95% CI=0.62-0.84) or being uninsured (aPR=0.60, 95% CI=0.41-0.88) were associated with decreased timeliness, compared to private insurance.
Conclusions: A limitation is that younger children at the pandemic's start (e.g., 0-6 months in March 2020) cannot be distinguished from older children (e.g., 7-13 months) in NIS-Child public data, and there may have been different impacts on vaccination timeliness for these subgroups. Within the context of this limitation, vaccination timeliness did not significantly differ between children surveyed at ages 19-23 months in the 2019 and 2021 NIS-Child.
