Background and objective:
During the Summer of 2022, many European countries started administering the fourth dose of Covid-19 vaccines upon developing an efficacious vaccine against the newly detected variants. However, despite governmental perspectives and previous findings reporting both actual and potential high acceptance rates on first-dose vaccination, more is needed to know about the diachronic evolution of the citizens engagement and how the change in perception of vaccine safety and efficacy affects vaccination intention. This study aims to investigate Italys fourth dose COVID-19 vaccination acceptance rates and how the differential presentation of positive-framed messages regarding the scientific evidence on the fourth dose safety and efficacy could influence citizen beliefs and responsiveness.

Methods:
We designed a multi-scale survey to study retrospectively the evolution of the beliefs behind each decision to vaccinate in the different stages of the vaccination campaign (i.e., from the first up to the third booster dose) and individual trust in scientific and political institutions in each period of the vaccination campaign. Secondarily, we implemented different message-framed vaccine safety and efficacy treatments to evaluate how they influence individual intention to uptake the new dose. Overall, theory-driven structural equation analysis will reveal what factors are significant predictors of the willingness to vaccinate in a balanced sample of 500 Italian participants.

Results:
We show a distinct evolution of the perceived efficacy and safety during the vaccine campaign and the importance of message targeting this latter component to increase vaccination intention. Moreover, we investigate how trust in political and scientific institutions changes during a pandemic according to vaccine beliefs.

Conclusion:
Vaccination campaign could benefit from messages based on vaccination safety more than efficacy perception and needs to consider a dynamic evolution of these components during multiple vaccination stages and the role of trust in scientific institutions more than governmental ones.