Seroprevalence of infection-induced SARS-CoV-2 antibodies among healthcare users of Northern Italy – results from two serosurveys (October-November 2019 – September-October 2021)

ABSTRACT

Objective To estimate the seroprevalence of SARS-CoV-2 in autumn 2019 (before case zero was identified in Italy) and 2021 among residual sera samples of healthcare users of Piedmont, Northwestern Italy. Methods Two serosurveys were conducted. Samples were tested for the detection of IgG antibodies against the S1 domain of the spike protein, using a semi-quantitative method. Positive samples from the 2019 survey were independently re-tested using a multiplex panel for the detection of IgG antibodies against the receptor-binding domain, S1 and S2, and nucleocapsid (N). Positive samples from the 2021 survey underwent repeat testing with ELISA for the detection of IgG anti-N antibodies. Prevalence rates according to gender and age groups, together with their respective 95% CIs, were calculated. Results Overall, the proportion of positive samples was 2/353 in 2019 and 22/363 in 2021, with an estimated seroprevalence of 0.27% (95% CI 0 – 1.86) and 6.21% (95% CI 3.9 – 9.31), in 2019 and 2021 respectively. Conclusion Results of this study support the hypothesis that the virus was circulating in Italy as early as autumn 2019. The role of these early cases in broader transmission dynamics remains to be determined.