*3.1. Participant Demographics and Exposures*

Between 1 March and 30 April 2020, 3609 individuals agreed to participate in the study. The mean number of screened individuals per administrative department was 721 (52–1430), representing 12 people per 100,000 inhabitants. The patients' demographics are outlined in Table 1.

Overall, 55.6% (2007/3609) were women and 44.4% were men (1602/3609). The median age was 51 years [interquartile range (IQR) 41–63], with the age group >55 years being most represented (41.4%; *n* = 1493/3609) and the 18–34 years group being the least represented (15.4%; *n* = 556/3609). All patients lived in the Lombardia and Liguria regions in the administrative departments of Varese (39.6%; *n* = 1430/3609), Pavia (24.1%; *n* = 871/3,609), Milano (21.2%; *n* = 764/3609), Genova (13.6%; *n* = 492/3609,) and Savona (1.4%; *n* = 52/3609;). Approximately 5.7% of the individuals (*n* = 207/3609) lived in a long-term care facility, whereas 5.0% (*n* = 179/3609) reported an occupational exposure to infected patients. When asked about recent medical history, 11.8% (*n* = 427/3609) reported symptoms consistent with influenza-like illness and 0.97% (*n* = 35/3609) reported loss of smell or taste within the previous month.


**Table 1.** Clinical characteristics of the study population.

#### *3.2. Prevalence of Sars-CoV-2 Antibodies*

Of the 3609 individuals included in the study population, 398 tested anti-SARS-CoV-2 positive [11.0% (CI 10.0%–12.1%)]. Seroprevalence was higher among women vs. men (12.5% vs. 9.2%, *p* = 0.002) and varied with age. The rate was highest among adults aged >55 years (13.2%), followed by adults aged 18–35 years (11.9%). As for geographical distribution, the highest prevalence of anti-Sars-COV-2 positivity was reported in the administrative departments of Savona (Figure 1). Table 2 shows estimated prevalence according to the three different laboratories.

**Figure 1.** Serologically-confirmed cases of SARS-CoV-2 in the general Italian population from 1 March to 30 April 2020. Red and blue lines represent the boarders of the Lombardia and Liguria regions, respectively. Percentages show the number of positive samples per number tested in each administrative department.
