J o u r n a l P r e -p r o o f Appendix 
It is widely known that the number of cases of COVID-19 who are reported to the surveillance systems largely underestimate the impact of the disease, since a large proportion of cases is not recognized either because they are asymptomatic, paucisymptomatic, or just not tested. Thus, seroprevalence studies are needed to estimate the number of people who have been exposed to SARS-CoV-2 in a specific area, to provide information on symptoms associated with infection, disease severity, and infection-fatality rate.
As reported by Havers et al. [1] , from several geographic sites the United States, 6 to 24 times more infections were estimated per site with seroprevalence than with COVID-19 case report data. A recent nationwide seroprevalence study in Spain showed remarkable geographical variation [2] . transmission to close contacts of COVID-19 cases were also investigated.
The study was conducted in 5 municipalities of the AP of Trento with the highest incidence of 
Blood samples (5 ml) were collected in serum separator tubes (BD Diagnostic Systems, Franklin
Lakes, NJ, USA) and centrifuged at room temperature at 1600 rpm for 10 min. Aliquots were transferred to 2ml polypropylene, screw cap cryo tubes (Sorfa, Zhejiang, China) and immediately frozen at -20 °C. Frozen sera were then shipped to the ISS lab in dry ice following biosafety shipment condition. Upon arrival serum samples were immediately stored at -80 °C.
Abbott SARS-CoV-2 IgG assays were performed on the Abbott Architect i2000SR automated analyzer 
The assay for molecular detection of SARS-CoV-2 on swabs was performed using the Abbott RealTime SARS-CoV-2 assay, on the Abbott m2000System. The SARS-CoV-2 primer and probe sets are designed to detect RNA from SARS-CoV-2 in naso/oropharyngeal swabs. Sample volume of 0.5 mL was extracted and a volume of 40 µL was used in the reaction.
Seroprevalence data were presented as proportions with 95% confidence interval (95% CI). 
Informed consensus for blood collection was obtained from all the participants. The study was approved by the Ethical Committee of the ISS (Prot. PRE BIO CE n.15997, 04.05.2020), Figure 2 ). Higher seropositivity was observed among males compared with females (24.4% vs 21.8%, respectively; p = 0.018).
The multivariable logistic regression model showed that age group, gender, municipality of residence, presence of symptoms, and working in contact with the public were associated with seropositivity ( A lower proportion of positive swabs in the municipalities with higher seroprevalence was observed, (i.e. in Canazei municipality; Figure 4 ).
The number of COVID-19 infected individuals reported by the Integrated surveillance data was underestimated, especially in the younger age groups (Table 3) . Overall, it was observed a ratio of 3.4
cases identified by serology test vs 1 by molecular test, with a maximum rate of about 7:1 in the age group <20 years and a minimum of about 1:1, in the age group >70 years of age. Overall, the IFR was 2.5%, ranging from 0 in those less than 20 years of age to 16.6% in the >70 years old group (data not shown).
We investigated the prevalence of anti-SARS-CoV-2 antibodies at the beginning of the reopening phase (the so-called "phase 2"), after the 2 months lockdown period, in the population of five municipalities with high incidence of COVID-19 located in an area of North east Italy.
Overall, the results indicate that almost one fifth of the population developed an antibody response, suggesting that they acquired the infection during the initial pandemic wave. The Abbott SARS-CoV-2
IgG test is reported to have a specificity of 100% (95% CI: 97.1-100.0%) and a sensitivity of 99.6% ]. An independent evaluation found a sensitivity of 100% and a specificity of 99.9% 17 days after appearance of symptoms [5] . However, analysis based on stored blood samples from PCR-positive cases found a sensitivity of 90% [6] . Moreover, test sensitivity may depend on infection severity, with lower antibody titers generally associated with younger age, and milder infections [7] , and sensitivity varying depending on time from onset of symptoms [8] .
When we recalculated estimates of prevalence adjusted for a test sensitivity of 90%, the prevalence increased homogeneously in each age group, without any specific and additional difference (Appendix, Serological surveys are the best tool to determine the spread of an infectious disease, particularly in the presence of asymptomatic cases or incomplete ascertainment of those with symptoms. Other SARS-CoV-2 seroprevalence studies have been conducted in different areas of the world [1, 2, [9] [10] [11] [12] showing that for every reported case the real number of infections in the community is higher. The relatively low seroprevalence observed in the context of an intense epidemic [1] [2] [9] [10] [11] [12] , with the consequence of a large susceptible population, emphasizes the need to maintain public health measures to avoid a new epidemic wave [13] [14] [15] .
In this study, geographical variation within the considered area was observed, with prevalence ranging approximately between 18% and 25%. We also found that the youngest age groups had higher seroprevalence, suggesting greater exposure to the virus but less susceptibility to the disease. However, the lower proportion of seropositive individuals in the age group >70 years can be partially explained by the fact that some elderly people with lab-confirmed result on swab died or were hospitalized, thus they did not participate in the study. In fact, 35 deaths have been registered among elderly during the study period. However, older people may have fewer social contacts due to more prudent behavior.
Male gender was associated with higher seroprevalence, which, however, does not completely explain the higher risk of developing full-blown, or even more severe disease.
Anosmia and ageusia were strongly associated with the presence of antibodies. As expected, other symptoms, such as fever and cough, were also more likely to occur among seropositive compared with seronegative individuals; however, the association was weaker, perhaps due to the concomitant occurrence of other viral illnesses, which may cause similar complaints.
The high estimate number of seropositive individuals compared with reported cases may be the consequence of the high number of persons who have mild or no illness or who do not seek medical care or testing, but who still contribute to virus transmission in the community.
When combined with local surveillance data, age-specific seroprevalence estimates can lead to robust estimates of the infection risk, which is crucial for the post-lockdown strategies. Differences in the antibody positivity among the five municipalities is likely due to the different time of sampling from the beginning of the spread of COVID-19 cases. Interestingly, seropositivity levels resulted to be higher in families with more components, suggesting a higher chance of getting infected in overcrowded settings or higher probability of multiple introductions into the household. Findings regarding the association between seroprevalence level and household size or presence of specific symptoms is consistent with those reported by other studies [16] [17] .
Before drawing conclusions, strengths and limits should be mentioned. Firstly, the refusal rate was low, thus the possibility of a selection bias was minimized. The information regarding the presence of symptoms eventually associated with the presence of antibodies was retrospectively collected, then a recall bias cannot be excluded. Moreover, other viruses might have co-circulated in the study area in the same period. Thirdly, reported symptoms may be underestimated since they were referred only to the month of April. This may have led to conservative estimates of the association between specific symptoms and seropositivity. However, the results suggest a high predictive value for specific symptoms such as anosmia. Finally, although the serological assay we used is assumed to have high sensitivity and specificity, the occurrence of some false positive results could not be completely ruled out.
In conclusion, we found a relatively high SARS-CoV-2 seroprevalence and an infection fatality rate of 2.5%, in a community with high incidence of COVID-19, with ski resort venues, located close to the border with the Lombardy Region, where devastating outbreak of COVID-19 had been reported. Our study confirms that reported cases of COVID-19 underestimate the prevalence of SARS-CoV-2 infection in the affected community, though they also show that herd immunity is far to be reached.
Serological studies are crucial to provide fundamental information for understanding the extent of past transmission and the current immunological state of the population. Repeated seroprevalence studies could provide further evidence on the transmission dynamics of SARS-CoV-2 in the population [15] .
Renzo, Manica Andrea, Fogarolli Angela E-healthcare Solution Service, APSS Trento; Maroni Veronica, Chizzola Elisa Privacy Office, APSS Trento; Sforzin Simona Primary Care Director APSS Trento, and the entire staff of primary care involved in the study.
Funds: APSS sustained the expenses for the IgG assays on collected sera. 
The authors declare no conflict of interest related to this study. 