Vaccine hesitancy, has become an increasingly important issue during the COVID-19 pandemic, to the extent that it was identified in 2019 by the World Health Organisation (1) as a major threat to global health. Due to the emergence of new variants, many international health agencies have already begun administering booster doses of the vaccine in response to these threats.

If vaccine acceptance has been a problem since the beginning of the pandemic, the administration of future booster shots could increase the hesitancy phenomenon, as studies have shown (2–8). As of 21st, July 2022 (at the time of writing this manuscript), only 107 million fully vaccinated people worldwide have received an additional vaccine dose or a booster dose, the highest level of protection against the virus.

Given these figures, the phenomenon of vaccine hesitancy will continue to be a serious threat to the end of the COVID-19 pandemic and for this reason it is necessary to investigate the mechanisms underlying this phenomenon by taking opportunity of large-scale vaccination due to the recent health emergency as a field of study.

Low vaccination intentions have been linked to people’s lack of trust in the safety of vaccines, complacency (seeing vaccination as largely unnecessary), calculativeness (carefully weighing risks and benefits), obstacles to vaccination, and low collective responsibility (e.g., unwillingness to get vaccinated to protect others) perceptions, according to previous research involving healthcare workers and the general population (9–12). Researchers have suggested a range of interventions, from informational campaigns to mandatory vaccination, for addressing these vaccine antecedents and boosting vaccination intentions (13–15). Offering incentives for vaccination could increase vaccination intentions in the same way that incentives have been demonstrated to encourage other healthy habits, such as keeping a healthier diet, stopping smoking, or doing physical exercise (16–18).

The literature (19–22) on this topic has emphasized the effectiveness of different types of incentive-based strategies to increase vaccination behaviours. Several studies (23–26) have shown how incentive-based strategies based on financial remuneration (e.g., monetary, bonus) in different countries have increased the acceptance of vaccines. Other studies (27–32) have also shown that the use of legal incentives linked to providing freedoms (e.g., the possibility to travel, the possibility to participate in public activities) are effective in promoting vaccination campaigns. However, vaccine hesitancy persists among certain population segments, necessitating further research into effective strategies for addressing this issue (33).

In this scenario, it is crucial to understand the effective approaches that can motivate the hesitant population to receive uptake doses of the COVID-19 vaccine, leveraging this health emergency as a field of study to gain a deeper understanding of vaccine hesitancy as a whole. Indeed, despite certain unique aspects related to COVID-19 (15), being against vaccine remains a significant barrier to COVID-19 vaccination (34). Therefore, effective strategies in this specific context may also prove effective in the future, presenting an opportunity to bridge the gap between scientific potential and citizen behavior.

Based on these premises, the purpose of the present study was to identify the correlation between different types of incentives (legal or financial) with people’s intentions to get a COVID-19 booster vaccine.

In this study, we investigated the effects of legal and financial incentives on COVID-19 booster dose vaccination intentions. Our results indicated that incentives are a suitable mean to motivate citizens to increase their willingness to get vaccinated even in the case of booster doses. Furthermore, we discovered that both types of incentives significantly relate with peoples’ willingness to vaccinate against COVID-19. However, for our sample, legal incentives—and in particular the introduction of vaccination health certificates required to access specific venues and being allowed to travel—were reported as the most effective incentive to boost vaccination intentions as indicated by other studies (25, 38, 39).

Our results are also in line with other studies that reported positive impacts of financial incentives on booster vaccination (23, 40, 41). Indeed, while vaccination mandates seem to be more likely to increase primary vaccination, incentives could be implemented to sustain booster uptake (42).

Indeed, as showed by other studies on the role of ethnicity in modifying the relationship between incentives and health behaviour change (43, 44), it is possible that for various populations and cultural backgrounds, the observed impacts of financial and legal incentives would differ. Thus, caution should be used when interpreting our findings. Additionally, several research advice considering the varying effects of rewards on persons with various motivations. According to the psychological literature, a person’s motivation levels may influence how they are influenced by the outside rewards that are given to them to increase their desire to carry out the requested behaviours (45–47). Evidence demonstrates that while typically highly motivated individuals are less influenced by external incentives (48), highly motivated individuals can occasionally be more susceptible to financial incentives than other individuals (49).

This study has some limitations, and results should be interpreted and used with caution. Firstly, the measures used in this study were self-reported and might be subject to reporting bias. In addition, the current study adopted a series of measures that were not validated—even if internal consistency was adequate. Second, as an observational cross-sectional study, causal relationships could not be inferred. Finally, there are indeed some socio-demographic variables that were not considered in this paper, and that is worth discussing. Despite its limitations, this study offers an important contribution to public policy literature and to policy makers in their efforts to explain and steer booster vaccination acceptance while facing an ongoing pandemic. Future research should explore the effects of other monetary and non-monetary types of incentives, as well as the interaction effect of incentive type and valence. Moreover other studies should estimate the impact of compulsory vaccine in acceptance rate and vaccine coverage (50, 51).

Based on results of this study, policymakers should consider incorporating common incentives into their vaccination promotion campaign, providing monetary incentives, and issuing health certifications—which permit access to public spaces and cultural events. Moreover, the social and cultural context of the intended vaccination target should be considered while designing these incentives.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by an independent ethics commission of the Department of Psychology of Università Cattolica del Sacro Cuore in Milan (CERPS). The patients/participants provided their written informed consent to participate in this study.

SB contributed to the conception and design of the study, methodology, and writing original draft. LP performed the statistical analysis. MA contributed to the methodology and preparation writing original draft. MP contributed to the methodology and data curation. GG supervised the research project and contributed to the writing, reviewing, and editing the manuscript. All authors contributed to the article and approved the submitted version.

This study was conducted within the CRAFT project, funded by Fondazione Cariplo and Regione Lombardia. The authors declare that the funders had no role in the study design, in interpretation of the results or in the decision to publish the results.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.