  Parental refusal and delay of childhood vaccination has increased in recent
years in the United States. This phenomenon challenges maintenance of herd
immunity and increases the risk of outbreaks of vaccine-preventable diseases.
We examine US county-level vaccine refusal for patients under five years of age
collected during the period 2012--2015 from an administrative healthcare
dataset. We model these data with a Bayesian zero-inflated negative binomial
regression model to capture social and political processes that are associated
with vaccine refusal, as well as factors that affect our measurement of vaccine
refusal.Our work highlights fine-scale socio-demographic characteristics
associated with vaccine refusal nationally, finds that spatial clustering in
refusal can be explained by such factors, and has the potential to aid in the
development of targeted public health strategies for optimizing vaccine uptake.
