Trends and risk factors of SARS-CoV-2 infection in asymptomatic blood donors

Background: A large proportion of SARS-CoV-2-infected individuals does not develop severe symptoms. Serological tests help in evaluating the spread of infection and disease immunization. The aim of this study was to prospectively examine the trends and risk factors of SARS-CoV-2 infection in blood donors. Study design and methods: We screened 8798 asymptomatic donors presenting in Milan from July 2020 to February 2021 (10,680 presentations) before the vaccination campaign for anti-nucleoprotein (NP) antibodies, and for anti-spike receptor-binding domain (RBD) antibodies and nasopharyngeal swab PCR in those who tested positive. Results: The prevalence of anti-NP+/RBD+ tests increased progressively with time up to ~15% (p <.0001), preceded by a peak of PCR+ tests. Anti-RBD titers were higher in anti-NP IgG+/IgM+ than in IgG+/IgM− individuals and in those with a history of infection (p <.0001); of these 197/630 (31.2%) displayed high titers (>80 AU/ml). Anti-RBD titers declined during follow-up, depending on baseline titers (p <.0001) and time (p =.025). Risk factors for seroconversion were a later presentation date and non-O ABO blood group (p <.001). A positive PCR was detected in 0.7% of participants in the absence of SARS-CoV-2 viremia. Conclusions: During the second wave of SARS-CoV-2 infection in Northern Italy, we detected an increase in seroprevalence in healthy blood donors from ~4% to ~15%, with a trend paralleling that observed in the general population. Seroconversion was more frequent in carriers of non-O blood groups. The persistence of anti-RBD antibodies was short-lived.