Objectives 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 seroprevalence screening. Methods We searched for SARS-CoV-2 IgGs in the entire voluntary population using lateral flow immune-cromatographic tests on capillary blood (rapid tests). We then performed chemioluminescent serological assays (CLIA) and naso-pharyngeal swabs in a randomized representative sample of 562 subjects and in every subject with a positive rapid test. Results Based on CLIA serologies on the representative random sample (509 subjects), we estimated a 23% IgG seroprevalence. We also found a strong correlation between age and prevalence, with the elderly showing the highest probability of a positive serological test. Conclusions In an area of unrestricted viral circulation less than one-fourth of the population tested positive for SARS-CoV-2 IgG. Seroprevalence increased with increasing age, possibly suggesting differences in susceptibility to the infection.