Predicting and Understanding Vaccine Hesitancy in Adults: Insights into Modifiable Risk Factors from a Mixed Methods Multiple Population Study Combining Machine Learning and Thematic Analysis

BackgroundVaccine hesitancy, the delay in acceptance or reluctance to vaccinate, ranks among the top threats to global health and undermines efforts to control vaccine-preventable diseases. The identification of modifiable risk factors and psychological processes underlying vaccine hesitancy is imperative to inform targeted interventions aimed at increasing vaccination uptake.MethodsLeveraging gradient boosting machines and thematic analysis, this mixed methods multiple population study aimed to identify modifiable predictors of vaccine hesitance in the general adult population. Predictors of vaccine hesitance was investigated in 2926 Norwegian adults (Mean age = 37.91, 79.69% female) during the COVID-19 pandemic, before the predictive utility of these variables was investigated in an independent sample of 734 adults in the United Kingdom (Mean age = 40.34, 57.08% female), enabling an examination of generalizability across populations. Two teams of authors independently conducted the machine learning and thematic analyses, blind to the analytic procedure and results of each other.ResultsThe machine learning model performed well in discerning vaccine hesitant (n = 248, 8.48% and n = 109, 14.85%, Norway and UK, respectively) from vaccine uptaking individuals (n = 2678, 91.52% and n = 625, 85.15%), achieving an AUC of 0.94 (sensitivity = 0.81; specificity = 0.98) in the Norwegian sample, and an AUC of 0.98 (sensitivity = 0.83; specificity = 0.97) in the out-of-sample replication in the UK. The mixed methods investigation identified five categories of modifiable risk factors and psychological processes tied to vaccine hesitancy, including illusion of invulnerability, doubts about vaccine efficacy, mistrust in official entities, minimization of the societal impact of COVID-19, and health-related fears tied to vaccination. Moreover, the media’s portrayal of rare incidents as fear amplifiers, and stigmatizing presentation of unvaccinated individuals were provided as additional underlying motives leading to vaccine reluctance and polarization. The thematic analysis further revealed information overload, fear of needles, previous negative vaccination experiences, fear of not getting healthcare follow-up after vaccination if needed, and vaccine aversion due to underlying (psychiatric) illness (e.g., eating disorders) as motives underlying vaccine hesitance.ConclusionsThe identified influential predictors were consistent across the two European samples, highlighting their generalizability across European populations. These predictors offer insights about modifiable factors that could be adapted by public health campaigns in mitigating misconceptions and fears related to vaccination toward increasing vaccine uptake. Moreover, the results highlight the media’s responsibility, as mediators of the public perception of vaccines, to minimize polarization and provide accurate portrayals of rare vaccine-related incidents, reducing the risk aggravating fear and reactance to vaccination.