Vaccine Hesitancy Under the Magnifying Glass: A Systematic Review of the Uses and Misuses of an Increasingly Popular Construct.

Resistance to vaccines has hindered attempts to contain and prevent outbreaks of infectious diseases for centuries. More recently, however, the term "vaccine hesitancy" has been used to describe not necessarily outright resistance but also a delay in acceptance or uncertainty regarding vaccines. Given concerns about hesitancy and its impact on vaccine uptake rates, researchers increasingly shifted the focus from resistance to vaccines toward vaccine hesitancy. Acknowledging the urgency to accurately assess the phenomenon, it is critical to understand the state of the literature, focusing on issues of conceptualization and operationalization. To carry out this systematic review, we collected and analyzed all published empirical articles from 2000 to 2021 that explicitly included quantitative self-report measures of vaccine hesitancy ( k  = 86). Using a mixed-method approach, the review demonstrates and quantifies crucial inconsistencies in the measurement of the construct, lack of clarity in regard to the determination of who should or should not be defined as hesitant, and overreliance on unrepresentative samples. Crucially, our analysis points to a potential systematic bias toward exaggerating the level of hesitancy in the population. Modeling a vaccine hesitancy co-citation network, the analysis also points to the existence of insular academic silos that make it harder to achieve a unified measurement tool. Theoretical and practical implications for academics, practitioners, and policymakers are discussed.