The observed variability in susceptibility to SARS-CoV-2 and severity of the ensuing COVID-19 have raised intense interest in their environmental and genetic risk factors. An early report from China
1 suggested that blood group A was associated with increased susceptibility and blood group O was associated with reduced susceptibility to SARS-CoV-2 infection. These reports motivated widespread interest in examining ABO blood groups as potential COVID-19 risk factors. Subsequent studies from Italy and Spain
2 reported that blood group A was associated with an increased risk of severe COVID-19 and blood group O was associated with a reduced risk. In contrast, a large Danish study
3 implicated disease susceptibility but not severity. However, observations from Boston, Massachussets,
4 and New York, New York,
5 did not confirm any specific associations between ABO blood group and disease. The controversy raised by these contrasting reports led to this case-control study.