Seroprevalence of the SARS-CoV-2 virus in the population of the southern Switzerland (Canton Ticino) – cohort study, results at 12 months

AIMS OF THE STUDY: A new emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified in Wuhan, China, in December 2019 and then spread rapidly, causing a global pandemic. In Europe, the first case was identified in Italy on 21 February 2020, in the Lombardy region bordering on the southern part of Switzerland (Canton Ticino), where 4 days later the first case was identified . Ticino was the most affected canton in Switzerland during the first wave of pandemic. In order to provide a reliable indicator for the spread of the virus in this region and help decision making at the public health level, a seroprevalence study of SARS-CoV-2 was conducted. METHODS: A cohort study was implemented on a randomly selected sample of 1500 persons. The sample is representative of the general population of the Canton of Ticino, stratified by sex and age from 5 years old.  Antibodies against the SARS-CoV-2 nucleocapsid protein were detected using a rapid qualitative test in 4 data collection periods over the course of 12 months (from May–June 2020 to May–June 2021). RESULTS: The seroprevalence of SARS-CoV-2 was estimated at 9.0% in spring 2020 (weeks 20–26), 8.4% in summer 2020 (weeks 32–38), 14.1% in autumn 2020 (weeks 45–52) and 22.3% in spring 2021 (weeks 18–23). In none of these four phases was evidence of an association between sex or specific age groups and presence of anti-SARS-CoV-2 antibodies detected. For risk factors, the only strong and significant association found was with diabetes in the first three data collection periods but not in the fourth. Among people who participated in all four phases of the study and tested positive anti-SARS-CoV-2 antibodies in the first test, 61.8% were still positive even in the fourth, 12 months later. CONCLUSIONS: The results support the hypothesis that, after one year and despite the severe burden in terms of hospitalisations and deaths experienced by the Canton Ticino, SARS-CoV-2 infection affected only a minority of the population (20%) and also suggest that the anti-nucleocapsid antibodies persist after 12 months in the majority of infected persons.