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Castiglione D'Adda is one of the municipalities more precociously and severely affected by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) epidemic in Lombardy. With our study we aimed to understand the diffusion of the infection by mass serological screening. We searched for SARS-CoV-2 IgGs in the entire population on a voluntary basis using lateral flow immunochromatographic tests (RICT) on capillary blood (rapid tests). We then performed chemioluminescent serological assays (CLIA) and naso-pharyngeal swabs (NPS) in a randomized representative sample and in each subject with a positive rapid test. Factors associated with RICT IgG positivity were assessed by uni- and multivariate logistic regression models. Out of the 4143 participants, 918 (22·2%) showed RICT IgG positivity. In multivariable analysis, IgG positivity increases with age, with a significant non-linear effect (p = 0·0404). We found 22 positive NPSs out of the 1330 performed. Albeit relevant, the IgG prevalence is lower than expected and suggests that a large part of the population remains susceptible to the infection. The observed differences in prevalence might reflect a different infection susceptibility by age group. A limited persistence of active infections could be found after several weeks after the epidemic peak in the area., Competing Interests: Medispa s.r.l. provided support in the form of salaries for authors RR and AP; Synlab s.p.a. provided support in the form of salaries for author CO. In particular none of the commercial funders (FC Internazionale Milano, Emporio Armani Olimpia Milano, SFD s.p.a. and Banca Mediolanum s.p.a.) had any role in data collection, analysis or intepretation; trial design; patient recruitment; or any aspect pertinent to the study, nor they have any commercial interest (e.g. consultancy, patents, products in development, marketed products, etc). This does not alter our adherence to PLOS ONE policies on sharing data and materials.