Background: to avoid a new spread of the SARS-CoV-2 infection, the post lockdown period requires the implementation of effective strategies for the case finding and contact tracing. The presence of asymptomatic subjects in the population, that are responsible for about 30% of the new infections, may complicate this phase. Serological tests for the measurement of immune response could represent an effective tool for the rapid monitoring of the population with asymptomatic infections and for estimating the proportion of immune in a territory, too.
Objectives: to describe the distribution of the immune response to SARS-CoV-2 infection in the population of the Municipality of Borgosesia (Piedmont Region, Northern Italy) and to estimate the efficacy of this strategy for the identification of asymptomatic cases.
Design: Cross-sectional study with administration of a rapid test to assess the seroprevalence of anti-SARS-CoV-2 IgG/IgM antibodies.
Setting and participants: all subjects resident in Borgosesia over the age of 18, where invited to participate. A rapid serological test was administered to enrolled participants to detect the presence of SARS-CoV-2 IgG and IgM antibodies on peripheral blood. Subjects with IgG or IgM positivity were offered to perform a swab test for viral RNA research.
Main outcome measures: the prevalence of IgM and IgG, and the relative risks of having positive swab test and of having symptoms similar to those of COVID-19 in the recent past has been estimated.
Results: 4,987 subjects participated to study, 44.5% of the adult population of Borgosesia. The average age was 55 years. There was a greater participation of women (54.4%), of people with a higher education level (37.3%) and of people without specific previous symptoms (95.1%). 245 people had a positive test for IgM or IgG, and the estimated prevalence was of 4.9%. 209 out of 245 subjects who were positive to the rapid test underwent to the RT-PCR test and this allowed to isolate 24 positive subjects.
Conclusions: the seroprevalence values estimated for subjects residing in the city of Borgosesia which underwent the rapid test for the detection of type M and type G antibodies on peripheral blood, confirmed the population-based estimates reported in literature, in particular with the results of the Italian survey of seroprevalence. Furthermore, the implementation of this test allowed the identification and isolation of completely asymptomatic subjects, that could have been identified only through screening with tests for viral RNA.
Keywords: COVID-19; SARS-CoV-2; immunity; seroprevalence; asymptomatic; population screening.