Stefano Porru, Maria Grazia Lourdes Monaco, Gianluca Spiteri, Angela Carta, Maria Diletta Pezzani, Giuseppe Lippi, Davide Gibellini, Evelina Tacconelli, Ilaria Dalla Vecchia, Emma Sala, Emanuele Sansone, Giuseppe De Palma, Carlo Bonfanti, Massimo Lombardo, Luigina Terlenghi, Enrico Pira, Ihab Mansour, Maurizio Coggiola, Catalina Ciocan, Alessandro Godono, Adonina Tardon, Marta-Maria Rodriguez-Suarez, Guillermo Fernandez-Tardon, Francisco-Jose Jimeno-Demuth, Rafael-Vicente Castro-Delgado, Tania Iglesias Cabo, Maria Luisa Scapellato, Filippo Liviero, Angelo Moretto, Paola Mason, Sofia Pavanello, Anna Volpin, Luigi Vimercati, Silvio Tafuri, Luigi De Maria, Stefania Sponselli, Pasquale Stefanizzi, Antonio Caputi, Fabriziomaria Gobba, Alberto Modenese, Loretta Casolari, Denise Garavini, Cristiana D’Elia, Stefania Mariani, Francesca Larese Filon, Luca Cegolon, Corrado Negro, Federico Ronchese, Francesca Rui, Paola De Michieli, Nicola Murgia, Marco Dell’Omo, Giacomo Muzi, Tiziana Fiordi, Angela Gambelunghe, Ilenia Folletti, Dana Mates, Violeta Claudia Calota, Andra Neamtu, Ovidiu Perseca, Catalin Alexandru Staicu, Angelica Voinoiu, Eleonóra Fabiánová, Jana Bérešová, Zora Kľocová Adamčáková, Roman Nedela, Anna Lesňáková, Jana Holčíková, Paolo Boffetta, Mahsa Abedini, Giorgia Ditano, Shuffield Seyram Asafo, Giovanni Visci, Francesco Saverio Violante, Carlotta Zunarelli, and Giuseppe Verlato
Background: The research aimed to investigate the incidence of SARS-CoV-2 breakthrough infections and their determinants in a large European cohort of more than 60,000 health workers. Methods: A multicentric retrospective cohort study, involving 12 European centers, was carried out within the ORCHESTRA project, collecting data up to 18 November 2021 on fully vaccinated health workers. The cumulative incidence of SARS-CoV-2 breakthrough infections was investigated with its association with occupational and social–demographic characteristics (age, sex, job title, previous SARS-CoV-2 infection, antibody titer levels, and time from the vaccination course completion). Results: Among 64,172 health workers from 12 European health centers, 797 breakthrough infections were observed (cumulative incidence of 1.2%). The primary analysis using individual data on 8 out of 12 centers showed that age and previous infection significantly modified breakthrough infection rates. In the meta-analysis of aggregated data from all centers, previous SARS-CoV-2 infection and the standardized antibody titer were inversely related to the risk of breakthrough infection (p = 0.008 and p = 0.007, respectively). Conclusion: The inverse correlation of antibody titer with the risk of breakthrough infection supports the evidence that vaccination plays a primary role in infection prevention, especially in health workers. Cellular immunity, previous clinical conditions, and vaccination timing should be further investigated.