Socioeconomic Barriers to COVID-19 Booster Vaccination in Southern Italy: A Retrospective Study to Evaluate Association with the Social and Material Vulnerability Index in Apulia

<b>Background</b>: Socioeconomic disparities may affect COVID-19 booster vaccination uptake, potentially undermining public health efforts. This study assessed the association between first booster dose coverage and municipal socioeconomic deprivation in the Apulia region of southern Italy. A secondary objective was to evaluate whether SARS-CoV-2 incidence modified this relationship. <b>Methods</b>: We conducted a retrospective observational study including Apulian residents aged ≥5 years from 1 January 2021, to 31 December 2022. First booster doses were identified using an algorithm based on dose chronology and national guidelines. Vaccination and infection data were retrieved from regional databases. Socioeconomic deprivation was measured using the Social and Material Vulnerability Index (SMVI) developed by the Italian National Institute of Statistics (ISTAT). Booster coverage was calculated at the municipal level. A multivariable Poisson's regression model was used to estimate the association between SMVI and booster uptake, adjusting for age group, primary vaccine type, SARS-CoV-2 incidence, and municipal vaccination rates. Analyses were stratified by sex. <b>Results</b>: A total of 2,732,258 individuals received a first booster dose. Booster coverage decreased with increasing SMVI. Among females, a significant reduction was observed in the highest deprivation category (RR > 102 vs. <99: 0.95; 95% CI: 0.94-0.97) and it was similar in males (RR: 0.95; 95% CI: 0.93-0.96). A significant interaction between age and deprivation was found in both sexes, with a sharper decline in younger individuals. Municipal vaccination rates were positively associated with booster uptake. SARS-CoV-2 incidence was positively associated with uptake only in males. <b>Conclusions</b>: The analysis revealed a significant association between lower socio-cultural level and lower adherence to the first booster dose of the COVID-19 vaccine. The decline is more pronounced among subjects younger than 50 years with high levels of vulnerability. The findings of this study suggest that to overthrow vaccine hesitancy, knowledge of the social setting allows for targeted communications to the different groups in the population. Further research is needed to define different approaches in the different social groups.