COVID-19 and ABO blood group: another viewpoint.

Abstract

Li et al.1 have recently published ‘Association between ABO blood groups and risk of SARS-CoV-2 pneumonia’, an observation already reported a few weeks ago as a MedRxiv preprint by Zhao et al.2 and which had a certain impact in the press. In both studies, the ABO blood groups distribution of patients with coronavirus disease 2019 (COVID-19) were compared to that of controls from the local populations that showed that blood group A was associated with an increased risk of infection, whereas group O was associated with a decreased risk. Considering this information rather as a working hypothesis, some scientists have called for caution.3 However, as already strongly suggested by others,4 this variable susceptibility to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection could be linked to circulating anti-A antibodies, which could interfere or even inhibit the virus–cell adhesion process. We had the idea to analyse these important available data series from the anti-A or -B antibodies viewpoint instead of ABO blood group antigens as the authors did. In fact, considering the largest series of patients with COVID-19 (N = 1888) analysed by Zhao et al.,2 we compared the proportion of those possessing anti-A in their serum (i.e. those of B and O blood groups) and those who did not (i.e. those of A and AB blood groups) to the control cohort (N = 3694; Table I). The results (Table I) indicate that subjects with anti-A in serum (i.e. B and O blood groups) are significantly less represented in the COVID-19 group than those lacking anti-A whatever the group: A and AB (P < 0·001), A (P < 0·001) or AB (P = 0·0323), whereas there was no significant difference versus circulating anti-B (data not shown). We then wondered if there was a difference between anti-A from O and anti-A from B, and then we compared the supposed protective effect of anti-A from O and from B (Table II). Whereas both blood group O and B patients possess circulating seric anti-A, it appears and it is statistically highly significant (P < 0·001) that O group patients are underrepresented (49·4 % vs. 57·6%), whereas B group patients are, on the contrary, overrepresented (50·6% vs. 42·4%), meaning that anti-A from O is more protective than anti-A from B. This latter observation is probably related to the fact that the immunoglobulin predominant isotype of anti-B/anti-A is IgM in serum from group A and B individuals, but IgG in O group serum, an already known notion,5 which has been well documented by flow cytometry.6 In conclusion, this way of analysing the data strongly suggests that the presence of anti-A antibodies in serum and more specifically IgG anti-A, should be considered as a factor more significant than the blood group itself, as far as the relationship between COVID-19 susceptibility and ABO blood groups is concerned. Far from intending to corroborate the authors' conclusions as such, we wanted to show that the resources of immuno-haematology allow several approaches that could perhaps be useful for the disease follow-up.