Increase in SARS-CoV-2 seroprevalence in healthy blood donors after the second wave of COVID-19 pandemic in South-Eastern Italy: evidence for asymptomatic young individuals as potential virus spreaders

Abstract: Background: Italy has been the first among western countries to experience SARS-CoV-2 spread during which the southern regions were also heavily affected by the pandemic. To understand and monitor properly the evolution of COVID-19 pandemic, population based seroprevalence studies are a valid tool for the infection rates and effective prevalence of the SARS-CoV-2. Aim: In this prospective study, we assessed the changes in SARS-CoV-2 seroprevalence rates among non-vaccinated blood donors in South-Eastern Italy over May 2020 to March 2021. Methods: 8,183 healthy blood donors referring to the Transfusion Center at the University Hospital Riuniti of Foggia (Italy) for blood donation in the period May 2020-March 2021 were tested for anti-SARS-CoV-2 antibodies by Ortho Clinical Diagnostics VITROS 3600. None of the considered subjects had a diagnosed symptomatic COVID-19 infection. Results: Overall, 516 resulted positive for anti-SARS-CoV-2 IgG antibodies (6.3%, 95% CI, 0.03-0.15%), 387 (4.7%) were male and 129 (1.7%) female. A statistically significant increase in the seropositive population was found from May 2020 to March 2021 (Fisher's p<0.001). The difference of the seroprevalence was significant in terms of age but not sex (2-sided p<0.05 for age; 2-sided p>0.05 for sex) in both groups. Conclusion: Our study shows a significant increase in the SARS-CoV-2 seroprevalence among blood donors and suggests a potential role of asymptomatic individuals in continuing the spread of the pandemic. These results may contribute to establishing containment measures and priorities in vaccine campaigns.