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BACKGROUND AND OBJECTIVES On June 4, 2021, Italy launched the coronavirus disease 2019 (COVID-19) vaccination of adolescents to slow down the COVID-19 spread. Although clinical trials have evaluated messenger ribonucleic acid (mRNA) vaccine effectiveness in adolescents, there is limited literature on its real-world effectiveness. Accordingly, this study aimed to estimate the effectiveness of mRNA COVID-19 vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and mild or severe COVID-19 in a cohort of Sicilian adolescents within a 6 month observation period. METHODS A retrospective cohort study was conducted with adolescents aged 12 to 18 years, residents of Sicily, who were followed from July 15 to December 31, 2021. SARS-CoV-2 infections, mild and severe COVID-19, and COVID-19-related intubation or deaths during the study period were compared between subjects vaccinated with 2 doses of mRNA vaccines and unvaccinated individuals. The Cox regression analysis, adjusted for age and sex, was performed to compare the 2 groups. RESULTS Overall, the study included a total at-risk population of 274 782 adolescents with 61.4% of them having completed the vaccination cycle by the end of the study. Unvaccinated subjects had higher incidence rates of SARS-CoV-2 (1043 × 10 000 vs 158.7 × 10 000 subjects), mild COVID-19 (27.7 × 10 000 vs 1.8 × 10 000 subjects) and severe COVID-19 (1.41 × 10 000 vs 0 subjects) compared with the vaccinated population. The estimated adjusted vaccine effectiveness against SARS-CoV-2 infection was 71.2%, reaching 92.1% and 97.9% against mild and severe COVID-19, respectively. CONCLUSIONS This study provides rigorous evidence of mRNA COVID-19 vaccines’ effectiveness in protecting adolescents from both SARS-CoV-2 infection and COVID-19.

Background: In Italy, the administration of the COVID-19 vaccine booster dose started on 27 September 2021, supported by clinical trials corroborating its efficacy. Given the paucity of real-world effectiveness data, this study aims to estimate the vaccine effectiveness of the booster dose against SARS-CoV-2 infection, severe disease, and death in the adult Sicilian population. Methods: This retrospective cohort study was carried out from 1 January to 31 March 2022 and included all residents in Sicily aged ≥ 18 years without previous SARS-CoV-2 infection and with a complete mRNA vaccine primary cycle. The cohort was split into two groups (booster and primary cycle) matched by age, gender, vaccine type, and month of completion of the primary vaccination cycle. Results: 913,382 subjects were observed in the study: 456,690 (50%) were vaccinated with two doses and 456,692 (50%) with three doses. There were 43,299 cases of SARS-CoV-2 among the two-doses vaccinees (9.5%) and 10,262 (2.2%) among the three-doses counterpart. Vaccine effectiveness in the booster cohort was 76.5% and 74.4% against SARS-CoV-2 infection, 85.7% and 79.7% against severe disease, and 84.1% and 73.1% against intubation or death, for BNT162b2 and mRNA-1273, respectively. Conclusions: This study confirmed the remarkable efficacy profile of the SARS-CoV-2 vaccine booster dose against infection, severe disease, and death attributable to the virus. Overall, the results of this study provide important real-world data to support the continued roll-out of the COVID-19 booster dose and have the potential to inform public health policy and guide decisions on vaccination strategies in countries around the world.