*4.6. COVID-19 and ABO Blood Group*

Here, we could show no clear influence of ABO blood group on outcome. The two fatal cases had blood group 0. Several studies showed an association between ABO blood groups and the risk of SARS-CoV-2 pneumonia [32].

Data from Wuhan (medRxiv, preprint, not peer-reviewed) including 2173 COVID-19 patients, showed that blood group A was overrepresented in COVID-19 compared with non-A blood groups. In contrast, patients with blood group O showed a significantly lower risk for the infection compared with non-O blood groups [33].

Similar results were shown by the Presbyterian hospital in New York, including 1559 patients with COVID-19 [34].

Gérard et al. studied the ABO blood group in patients with COVID-19, by comparing the patients (n = 1888) 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) [35]. They found significantly less COVID-19 in patients with anti-A in serum (i.e., B and O blood groups) compared to those lacking anti-A (i.e., A and AB blood groups), showing a possible protective effect of anti-A. Surprisingly, Gérard et al. also found a difference between anti-A from O and anti-A from B. The anti-A from O showed an underrepresentation in COVID-19 and anti-A from B an overrepresentation, indicating that anti-A from O is more protective than anti-A from B [35]. This important difference could be related to the isotype of antibodies, being anti-A isotype IgM in serum from blood group B patients, but IgGs in blood group O serum. Although several studies have shown a higher risk of SARS-CoV-2 infection for certain blood groups, one meta-analysis has shown that there was no clear correlation between blood groups and the severity of COVID-19 [36].
