SARS-CoV-2 seroprevalence trends in healthy blood donors during the COVID-19 outbreak in Milan

• Published in: Blood transfusion = Trasfusione del sangue Vol. 19; no. 3; pp. 181 - 189
• Main Authors: Valenti, Luca, Bergna, Annalisa, Pelusi, Serena, Facciotti, Federica, Lai, Alessia, Tarkowski, Maciej, Lombardi, Angela, Berzuini, Alessandra, Caprioli, Flavio, Santoro, Luigi, Baselli, Guido, Ventura, Carla Della, Erba, Elisa, Bosari, Silvano, Galli, Massimo, Zehender, Gianguglielmo, Prati, Daniele
• Format: Journal Article
• Language: English
• Published: Italy Edizioni SIMTI - SIMTI Servizi Srl 01-05-2021
• Subjects: Adult; Age Factors; Antibodies, Viral - blood; Asymptomatic Infections - epidemiology; Bayes Theorem; Blood Donation And Donor Infectious Disease Testing; Blood Donors - statistics & numerical data; Confidence Intervals; COVID-19 - epidemiology; COVID-19 - immunology; COVID-19 Serological Testing - methods; Cross-Sectional Studies; Female; Humans; Immunoglobulin G - blood; Immunoglobulin M - blood; Italy - epidemiology; Male; Middle Aged; Pandemics; Regression Analysis; Risk Factors; SARS-CoV-2 - immunology; Seroconversion; Seroepidemiologic Studies; Spike Glycoprotein, Coronavirus - immunology; Italy
• ISSN: 1723-2007; 2385-2070
• DOI: 10.2450/2021.0324-20

The Milan metropolitan area in Northern Italy was among the most severely hit by the SARS-CoV-2 outbreak. The aim of this study was to examine the seroprevalence trends of SARS-CoV-2 in healthy asymptomatic adults, and the risk factors and laboratory correlates of positive tests.
We conducted a cross-sectional study in a random sample of blood donors, who were asymptomatic at the time of evaluation, at the beginning of the first phase (February 24
to April 8
2020; n=789). Presence of IgM/IgG antibodies against the SARS-CoV-2-Nucleocapsid protein was assessed by a lateral flow immunoassay.
The test had a 100/98.3 sensitivity/specificity (n=32/120 positive/negative controls, respectively), and the IgG test was validated in a subset by an independent ELISA against the Spike protein (n=34, p<0.001). At the start of the outbreak, the overall adjusted seroprevalence of SARS-CoV-2 was 2.7% (95% CI: 0.3-6%; p<0.0001 vs 120 historical controls). During the study period, characterised by a gradual implementation of social distancing measures, there was a progressive increase in the adjusted seroprevalence to 5.2% (95% CI: 2.4-9.0; 4.5%, 95% CI: 0.9-9.2% according to a Bayesian estimate) due to a rise in IgG reactivity to 5% (95% CI: 2.8-8.2; p=0.004 for trend), but there was no increase in IgM
(p=not significant). At multivariate logistic regression analysis, IgG reactivity was more frequent in younger individuals (p=0.043), while IgM reactivity was more frequent in individuals aged >45 years (p=0.002).
SARS-CoV-2 infection was already circulating in Milan at the start of the outbreak. The pattern of IgM/IgG reactivity was influenced by age: IgM was more frequently detected in participants aged >45 years. By the end of April, 2.4-9.0% of healthy adults had evidence of seroconversion.