Amati, Rebecca Piumatti, Giovanni Franscella, Giovanni Buttaroni, Peter Camerini, Anne-Linda Corna, Laurie Levati, Sara Fadda, Marta Fiordelli, Maddalena Annoni, Anna Maria Bezani, Kleona Amendola, Antonio Fragoso Corti, Cristina Sabatini, Serena Kaufmann, Marco Frei, Anja Puhan, Milo Alan Crivelli , Luca Albanese, Emiliano 2023 The COVID-19 pandemic continues, and evidence on infection- and vaccine-induced immunity is key. We assessed COVID-19 immunity and the neutralizing antibody response to virus variants across age groups in the Swiss population. Study Design: We conducted a cohort study in representative community-dwelling residents aged five years or older in southern Switzerland (total population 353,343), and we collected blood samples in July 2020 (in adults only, N = 646), November–December 2020 (N = 1457), and June–July 2021 (N = 885). Methods: We used a previously validated Luminex assay to measure antibodies targeting the spike (S) and the nucleocapsid (N) proteins of the virus and a high-throughput cell-free neutralization assay optimized for multiple spike protein variants. We calculated seroprevalence with a Bayesian logistic regression model accounting for the population’s sociodemographic structure and the test performance, and we compared the neutralizing activity between vaccinated and convalescent participants across virus variants. Results: The overall seroprevalence was 7.8% (95% CI: 5.4–10.4) by July 2020 and 20.2% (16.4–24.4) by December 2020. By July 2021, the overall seroprevalence increased substantially to 72.5% (69.1–76.4), with the highest estimates of 95.6% (92.8–97.8) among older adults, who developed up to 10.3 more antibodies via vaccination than after infection compared to 3.7 times more in adults. The neutralizing activity was significantly higher for vaccine-induced than infection-induced antibodies for all virus variants (all p values < 0.037). Conclusions: Vaccination chiefly contributed to the reduction in immunonaive individuals, particularly those in older age groups. Our findings on the greater neutralizing activity of vaccine-induced antibodies than infection-induced antibodies are greatly informative for future vaccination campaigns. application/pdf https://susi.usi.ch/global/documents/331793 https://n2t.net/ark:/12658/srd1331793 https://susi.usi.ch/documents/331793/files/Albanese_2023_MDPI_ijerph_Trajectories of seroprevalence.pdf eng info:eu-repo/semantics/altIdentifier/doi/10.3390/ijerph20043703 info:eu-repo/semantics/altIdentifier/ark/12658/srd1331793 info:eu-repo/semantics/openAccess CC BY International Journal of Environmental Research and Public Health. - 2023, vol. 20, p. 3703 COVID-19 Antibodies Vaccination Cohort study Seroprevalence Acquired immunity info:eu-repo/classification/udc/61 Trajectories of seroprevalence and neutralizing activity of antibodies against SARS-CoV-2 in Southern Switzerland between July 2020 and July 2021 : an ongoing, prospective population-based cohort study http://purl.org/coar/resource_type/c_6501