Prevalence of SARS-CoV-2 IgG antibodies in an area of North-eastern Italy with a high incidence of COVID-19 cases: a population-based study

Stefanelli, Paola; Bella, Antonino; Fedele, Giorgio; Pancheri, Serena; Leone, Pasqualina; Vacca, Paola; Neri, Arianna; Carannante, Anna; Fazio, Cecilia; Benedetti, Eleonora; Fiore, Stefano; Fabiani, Concetta; Simmaco, Maurizio; Santino, Iolanda; Zuccali, Maria Grazia; Bizzarri, Giancarlo; Magnoni, Rosa; Benetollo, Pier Paolo; Merler, Stefano; Brusaferro, Silvio; Rezza, Giovanni; Ferro, Antonio

OBJECTIVES: A seroprevalence study of SARS-CoV-2 was conducted in a high-incidence area located in North-eastern Italy. METHODS: All citizens above ten years of age resident in 5 municipalities of the Autonomous Province of Trento, with the highest incidence of COVID-19 cases, were invited to participate in the study. Overall, among 6098 participants, 6075 sera and a standardized questionnaire administered face-to-face were collected between May 5 and 15, 2020 and examined. Symptomatic individuals and their family contacts were tested by RT-PCR. Anti-SARS-CoV-2 antibodies were detected using an Abbott SARS-CoV-2 IgG assay which was performed on the Abbott Architect i2000SR automated analyzer. Seroprevalence was calculated as the proportion of positive people on the total number of tested. A multivariable logistic regression model was performed to assess the relationship between seropositive versus seronegative individuals for a set of explanatory variables. RESULTS: A total of 1402 participants were positives for IgG antibodies against SARS-CoV-2, with a prevalence of 23.1% (1402/6075). The highest prevalence was found in the age class 40-49 years. Overall, 34.4% (2096/6098) of the participants reported at least 1 symptom. The ratio between reported cases identified by molecular test and those resulting seropositive was 1:3, with a maximum ratio of about 1:7 in the age group <20 years and a minimum around 1:1 in those >70 years old. The infection fatality rate was 2.5% (35/1402). Among the symptoms, anosmia and ageusia were strongly associated with seropositivity. CONCLUSIONS: The estimated seroprevalence of 23% was 3-fold higher than the number of cases reported in the COVID-19 Integrated Surveillance data in the study area. This may be explained in part by a relatively high number of individuals presenting mild or no illness, especially of younger age, and/or who did not seek medical care or testing, but who may contribute to virus transmission in the community.