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Reduced prevalence of SARS-CoV-2 infection in ABO blood group O

Our take —

This population-based retrospective study from Denmark utilized existing health records, blood group information, and SARS-CoV-2 testing data to suggest a moderate difference in risk of SARS-CoV-2 infection among those with blood groups O (decreased risk) and A (increased risk). However, the strength of the blood group and SARS-CoV-2 susceptibility relationship remains unclear. Furthermore, as the prevalence of blood group types can differ significantly by genetic ancestry or ethnicity, additional work is needed to characterize the risk of blood group antigens on SARS-CoV-2 risk in non-European populations.

Study design

Retrospective Cohort

Study population and setting

Leveraging electronic health record data from all Danish patients and other centralized databases, this retrospective cohort analysis explored the ABO and RhD blood group distributions among 473,654 individuals tested by real-time polymerase chain reaction (PCR) for SARS-CoV-2 who had blood group information available (out of 841,327 individuals tested between 2/27/2020 – 7/30/2020). These mostly symptomatic (74%) individuals were compared with a reference group of 2,204,742 non-tested individuals from across Denmark who had blood group information available. In addition to PCR-based testing results, demographic and COVID-19 clinical outcomes including death were explored for associations with blood group.

Summary of Main Findings

There were no significant differences observed between RhD or ABO blood groups for hospitalization or death due to COVID-19. However, individuals with blood group O were less likely to have a positive SARS-CoV-2 test (RR 0.87) and those with blood group A were more likely to have a positive SARS-CoV-2 test (RR 1.09) overall. These analyses did not adjust for the changing testing strategy throughout the early course of the pandemic in Denmark.

Study Strengths

A largely ethnically homogenous population may minimize potential bias introduced from differences in distributions of blood types by ethnicity, which may also be associated with exposure to the virus or health outcomes due to socioeconomic histories . Further, Denmark has free and universal healthcare services which are linked to a set of exceptional centralized data repositories used to conduct the analysis . Given the large sample size, this study was well powered to detect differences in COVID-19 testing positivity among those with blood groups A, B, and O.


While the authors attempted to account for potential confounding due to ancestry-related differences in blood group type prevalence through an adjusted analysis, ethnic population-specific data on the counts of non-Western individuals are not provided (e.g., stratified results) and would be needed to confirm the authors findings. Temporally stratified results are not provided, but coupled with ethnicity-specific testing counts may be of potential importance given the testing limitations during these stages of the pandemic. We stress the importance of ethnicity-specific results because many non-European populations have a lower prevalence of O group antigen, and non-western individuals were disproportionately identified as cases (18%) compared to comprising 9% of the Danish reference population.

Value added

Blood group type has been associated with the susceptibility to multiple infectious diseases and this study provides important preliminary findings within the Danish population related to the potential association between blood groups A (increased risk) and O (decreased risk) and the risk of SARS-CoV-2 infection.

This review was posted on: 19 December 2020