SHORT COMMUNICATION Role of ABO blood system in COVID-19: Findings from a southern Italian study Pierpaolo Negro 1 | Maurizio Congedo 2 | Antonella Zizza 3 | Marcello Guido 4 | Gianfranco Sacquegna 1 | Giuseppe Pulito 5 | Giambattista Lobreglio 6 1 Inter-company Department of Transfusion Medicine (IDTM) of Local Health Unit (LHU) of Lecce, Immunohaematology and Transfusion Medicine Unit, Vito Fazzi Hospital, Lecce, Italy 2 Dermatology Unit, Vito Fazzi Hospital, Lecce, Italy 3 Institute of Clinical Physiology, National Research Council, Lecce, Italy 4 Laboratory of Hygiene, Department of Biological and Environmental Sciences and Technologies, University of Salento, Lecce, Italy 5 Intensive Care and Resuscitation Unit, Vito Fazzi Hospital, Lecce, Italy 6 Clinical Pathology and Microbiology, Vito Fazzi Hospital, Lecce, Italy Correspondence Pierpaolo Negro, Inter-company Department of Transfusion Medicine (IDTM) of Local Health Unit (LHU) of Lecce, Immunohaematology and Transfusion Medicine Unit, Vito Fazzi Hospital, Lecce, Italy. Email: pierpaolonegro.pn@gmail.com Abstract Background: COVID-19 is a worldwide infection caused by SARS-CoV-2 and infects humans by binding to the ACE2 receptor. Blood group ABO glycoproteins can influ- ence the binding of the virus to ACE2. The role of ABO blood system in the suscepti- bility to infection as well as in the clinical outcome of infected patients is still controversial and needs to be clarified. Methods: We conducted a retrospective study of 167 patients positive for SARS- CoV-2 who underwent nasopharyngeal swab, and of a control group represented by 891 subjects negative for SARS-CoV-2, to assess the association between ABO and Rh blood system and occurrence of SARS-CoV-2 infection, clinical presentation, and outcome of disease. Results: In the cohort of patients positive for SARS-CoV-2, no statistically significant difference in the distribution of ABO blood types compared with controls was observed. Patients with blood type A had a higher risk of developing symptomatic disease (p = 0.002; odds ratio [OR = 3.592]; 95% confidence interval [CI] = 1.576– 8.187) compared to patients with blood types B, AB, and O. Patients with blood types B (p = 0.021; OR = 0.293; 95%CI = 0.099–0.869) and O (p = 0.018; OR = 0.417; 95%CI = 0.199–0.871) showed a lower risk in comparison to the other groups. The clinical progression to mild/moderate and severe/critical disease and the mortal- ity showed no association. Moreover, no relationship with Rh blood type was found. Conclusions: Our findings support a role of ABO blood type in the development of symptomatic disease with a higher risk in subjects with blood type A and a protective effect of blood types B and O. Blood types do not seem, however, to play a role in susceptibility, progression to severe disease, and death. KEYWORDS ABO blood type, COVID-19, Rh blood type, SARS-CoV-2 Pierpaolo Negro, Maurizio Congedo, and Antonella Zizza have contributed equally to this work and should be considered joint first authors. Received: 12 October 2020 Accepted: 10 February 2021 DOI: 10.1111/tme.12797 Transfusion Medicine. 2021;1–5. wileyonlinelibrary.com/journal/tme © 2021 British Blood Transfusion Society. 1