Higher levels of von Willebrand factor in hospitalised patient plasma provides an explanation for the association of ABO blood group and secretor status with COVID19 severity.

Early studies of COVID19 in Wuhan suggested a relationship between SARS-CoV-2 infection and an individual's ABO blood group with a higher frequency of infected blood group A patients than would be expected if ABO group had no influence on infection status. 1 Since then numerous additional studies have corroborated this observation, whilst some have found no link at all. 2 – 4 Possible explanations for this discrepancy may be differences in the frequency of different blood groups within different populations and the presence of underlying disease in infected hospitalised patients when compared with healthy individuals. In a study of COVID19 patients in Bristol, UK, we observed a significant association between COVID19 hospitalisation, blood group A patients, who are secretors, and cardiovascular compli-cations. 5 Other studies have also reported an association between blood groups, cardiovascular events and COVID19. 6 There is a well described link between blood group “ non-O " (A, B, AB) and throm-botic events centred on higher circulating levels of von Willebrand factor (VWF) in non-O individuals when compared with those of group O. 7 This may be because additional glycosylation of an H anti-gen, spatially close to amino-acid 1574 within VWF, to form either the A or B antigen, provides protection against proteolytic degrada-tion by the VWF regulator protease ADAMTS13. 7 Fucosylation of a terminal galactose to form the H-antigen on secreted proteins is per-formed by Fucosyltransferase 2 (FUT2) and individuals with a functional FUT2 gene express ABO on non-haematological cells and on secreted proteins (such as VWF) and are termed secretors. However, (cid:1) 20% of the population have an inactivating mutation in FUT2 , con-sequently they only express ABO on haematological cells, and are