BackgroundUnderstanding lay perceptions concerning vaccination and identifying the most important psychological determinants of vaccine acceptability are relevant for health promotion campaigns. MethodsThis cross-sectional study aimed to identify the psychological determinants of HPV vaccine and seasonal flu vaccine acceptability in a national sample of young adults. An extended version of the health belief model (HBM) provided the main theoretical framework. Four hundred and one adults aged 18–26 (79% women), completed the theory-based survey and were included in analyses. The main outcomes were intentions to vaccinate against HPV and the seasonal flu. Descriptive statistics, bivariate correlations and hierarchical multiple regression were used for data analysis. ResultsThe models predicted 51% and 60% of the variability in intentions to vaccinate against HPV and the seasonal flu, respectively. For both decisional contexts, anticipated inaction regret (assuming one were unvaccinated and later contracted infection) was the strongest predictor of intentions, contributing above and beyond the role of traditional risk belief constructs (HBM constructs). Other shared predictors of intentions were perceived effectiveness of vaccine, perceived vaccine safety, perceived susceptibility to disease and previous vaccine refusal. The study also provides insights into young adults’ vaccine-related knowledge and their information-seeking practices. ConclusionFindings outline directions for future research and implications for health communication campaigns.