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


Luca Valenti, Annalisa Bergna, Serena Pelusi, Federica Facciotti, Alessia Lai, Maciej Tarkowski, Alessandra Berzuini, Flavio Caprioli, Luigi Santoro, Guido Baselli, Carla Della Ventura, Elisa Erba, Silvano Bosari, Massimo Galli, Gianguglielmo Zehender, Daniele Prati


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, the risk factors, and laboratory correlates.


We conducted a cross-sectional study in a random sample of blood donors since the start of the outbreak (February 24th to April 8th 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, and for IgG+ was validated in a subset by an independent ELISA against the Spike protein (N=34, P<0.001). At the outbreak start, the overall adjusted seroprevalence of SARS-CoV-2 was 2.7%, 95% c.i. 0.3-6% (P<0.0001 vs. 120 historical controls). During the study period characterized by a gradual implementation of social distancing measures, there was a progressive increase in adjusted seroprevalence to 5.2%, 95% c.i. 2.4-9.0, due to a rise in IgG+ tests to 5%, 95%CI 2.8-8.2 (P=0.004 for trend, adjusted weekly increase 2.7+/-1.3%), but not of IgM+ (P=NS). At multivariate logistic regression analysis, seroconversion to IgG+ was more frequent in younger (P=0.043), while recent infections (IgM+) in older individuals (P=0.002). IgM+ was independently associated with higher triglycerides, eosinophils, and lymphocytes (P<0.05).


SARS-CoV-2 infection was already circulating in Milan at the outbreak start. Social distancing may have been more effective in younger individuals, and by the end of April 2.4-9.0% of healthy adults had evidence of seroconversion. Asymptomatic infection may affect lipid profile and blood count.

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