The actual effectiveness of the still-to-come vaccination against the coronavirus SARS-CoV-2 might be challenged by vaccine hesitancy, a rather common and known phenomenon whose psychological predictors are, nevertheless, still largely debated. Our study aims at understanding how adult citizens' health engagement, perceived COVID-19 susceptibility and severity, and general vaccine-related attitudes affect the willingness to vaccinate against COVID-19. To that end, on a sample of Italian citizens, we implemented a path model to test the impact of health engagement on the willingness to vaccinate against SARS-CoV-2, and whether this relationship is direct or mediated by the general attitude towards vaccines and the risk perception. Moreover, we tested the configural and weak invariance of the model across gender and three age groups. Results show that health engagement is positively related to the intention to vaccinate and that this relationship is partially mediated by the general attitude towards vaccines. The model appears invariant across genders and partially invariant across age groups, showing some differences in the role of perceived susceptibility. These findings vouch for the implementation of educational campaigns aimed at sustaining future vaccination programs that also include health engagement promotion.