Project Summary/Abstract
Delivery of early childhood vaccines using an established schedule is a key strategy for infectious disease
control in the United States. However, between 50% and 75% of U.S. children experience undervaccination,
defined as either getting some or all recommended vaccines late or not getting them at all, before their 2nd
birthday. Outbreaks of measles, pertussis, and other vaccine-preventable diseases have been linked to
children who were behind on or missing vaccines, either due to parental vaccine hesitancy or other barriers to
accessing immunization services. There are currently no systematic, population-based approaches for
tracking adherence to the U.S. recommended immunization schedule, or the prevalence of parents who
choose to deviate from the recommended schedule. Furthermore, standard measures are inadequate for
identifying the reasons why we fail to achieve vaccine coverage goals, which is a particular problem in
populations that experience immunization disparities, including children living in the rural U.S. For this project,
we will analyze 11 years of public use data collected in the National Immunization Survey-Child (NIS-Child) to
quantify four novel measures of early childhood immunization services quality: (1) vaccine timeliness, (2)
immunization schedule adherence, (3) undervaccination due to parental vaccine hesitancy, and (4)
undervaccination due to other access barriers. In our analyses of this nationally representative data source,
we will identify trends in and factors associated with these measures in the U.S. overall in 2011-2019 (Specific
Aim 1a), for each U.S. state in 2011-2019 (Specific Aim 1b), and assess how the COVID-19 pandemic affected
these trends in 2020-2021 (Specific Aim 1c). We will use mixed effects log-binomial models and zero-inflated
Poisson models, as well as piecewise regression methods, to determine how time, socioeconomic,
demographic, and other factors have affected these immunization quality measures in and across the U.S.
Then, we will access restricted use NIS-Child data to determine how trends in and factors associated with
immunization schedule adherence and specific undervaccination patterns are modified by rurality (Specific Aim
2). Our proposed analyses, along with our dissemination efforts, will contribute to the long-term goal of
establishing methods for routine surveillance of these early childhood immunization services quality measures
at national, state, and some local levels. Furthermore, this study will establish new data-driven methods for
distinguishing between undervaccination due to parental vaccine hesitancy versus other healthcare access
barriers; an important distinction, since interventions to address these two types of barriers fundamentally
differ. The results from this project, and particularly the examinations of effect modification by rurality, will
inform targeted interventions for increasing on-time vaccination and immunization schedule adherence,
decreasing vaccine-preventable disease risk, and achieving immunization coverage equity in early childhood.
