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

Covid-19 Donors Study (CoDS) network (Appendix 1), Luca Valenti, Annalisa Bergna, Serena Pelusi, Federica Facciotti, Alessia Lai, Maciej Tarkowski, Angela Lombardi, Alessandra Berzuini, Flavio Caprioli, Luigi Santoro, Guido Baselli, Carla Della Ventura, Elisa Erba, Silvano Bosari, Massimo Galli, Gianguglielmo Zehender, Daniele Prati

Research output: Contribution to journalArticlepeer-review


BACKGROUND: 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. MATERIALS AND METHODS: 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 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. RESULTS: 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). DISCUSSION: 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.

Original languageEnglish
Pages (from-to)181-189
Number of pages9
JournalBlood transfusion = Trasfusione del sangue
Issue number3
Publication statusPublished - May 1 2021

ASJC Scopus subject areas

  • Immunology and Allergy
  • Hematology


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