State-Level Variations and Factors Associated with Adult Vaccination Coverage: A Multilevel Modeling Approach

Background Adult vaccination rates in the USA are generally low and fall short of public health goals. Objectives Our aim was to evaluate the effect of state-level characteristics on adult vaccination coverage in the USA. Methods This study was a cross-sectional, retrospective analysis of 2015-2017 Behavioral Risk Factor Surveillance System data, conducted from March to October 2019 and including seasonal influenza; pneumococcal; tetanus, diphtheria, and acellular pertussis (Tdap); and herpes zoster (HZ) vaccines. Multilevel logistic regression models examined interstate vaccination coverage variability and assessed the impact of state-level characteristics, with model-adjusted coverage estimated. Results Model-adjusted vaccination coverage varied by state, with 35.1-48.1% coverage for influenza (2017), 68.2-80.8% for pneumococcal (2017), 21.9-46.5% for Tdap (2016), and 30.5-50.9% for HZ (2017). Characteristics associated with vaccination included state-level insurance coverage, pharmacists' vaccination authority, vaccination exemptions, and adult immunization information systems participation, as well as individual-level measures of income and education. After adjusting for these factors, substantial interstate heterogeneity remained. Conclusions Model-adjusted coverage was generally low and varied by state. A small number of state-level characteristics partially explained interstate coverage variability. This and future research assessing additional state characteristics may help determine policies most likely to increase adult vaccination. Plain Language Summary Adult vaccination rates in the USA are generally low and fall short of public health goals. Previous studies have indicated that adult vaccination rates vary between states and that individual characteristics affect vaccination coverage. We used modeling to evaluate the effects of both individual- and state-level factors on adult vaccination coverage. Health insurance coverage, the authority of pharmacists to vaccinate, existence of vaccination exemptions, and immunization information systems adult participation rates had a positive impact on vaccination coverage, although the impact varied by vaccine. These results provide policy decision makers at both state and federal levels with information to consider when expanding vaccination programs or preventive care efforts. However, additional data are needed to further explain the variations between states.