Introduction: 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. Methods: We searched for SARS-CoV-2 IgGs in the 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 multivariable logistic regression models. Results: Out of the 4143 participants, 918 (22·2%) showed an 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. Conclusions: 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 (positive NPSs) could be found after several weeks after the epidemic peak in the area. Funding Statement: CISOM (Corpo Italiano di Soccorso dell'Ordine di Malta), FC Internazionale Milano, SFD s.p.a , Emporio Armani Olimpia Milano through donations to the Dipartimento di Scienze Biomediche e Cliniche (DIBIC) of the University of Milan and to Banca Mediolanum through donation to ASST FatebenefratelliSacco, Milano. Declaration of Interests: The authors declare no conflicts of interest. Ethics Approval Statement: The study was approved by University of Milan’s Ethical Committee.