• Joel G Ray, Michael J Schull, Marian J Vermeulen, and Alison L Park.
• St. Michael's Hospital, University of Toronto, and ICES, Toronto, Ontario, Canada (J.G.R.).
• Ann. Intern. Med. 2021 Mar 1; 174 (3): 308-315.

BackgroundThe ABO and rhesus (Rh) blood groups may influence risk for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.ObjectiveTo determine whether ABO and Rh blood groups are associated with risk for SARS-CoV-2 infection and severe coronavirus disease 2019 (COVID-19) illness.DesignPopulation-based cohort study.SettingOntario, Canada.PatientsAll adults and children who had ABO blood group assessed between January 2007 and December 2019 and who subsequently had SARS-CoV-2 testing between 15 January and 30 June 2020.MeasurementsThe main study outcome was SARS-CoV-2 infection, determined by viral RNA polymerase chain reaction testing. A second outcome was severe COVID-19 illness or death. Adjusted relative risks (aRRs) and absolute risk differences (ARDs) were adjusted for demographic characteristics and comorbidities.ResultsA total of 225 556 persons were included, with a mean age of 54 years. The aRR of SARS-CoV-2 infection for O blood group versus A, AB, and B blood groups together was 0.88 (95% CI, 0.84 to 0.92; ARD, -3.9 per 1000 [CI, -5.4 to -2.5]). Rhesus-negative (Rh-) blood type was protective against SARS-CoV-2 infection (aRR, 0.79 [CI, 0.73 to 0.85]; ARD, -6.8 per 1000 [CI, -8.9 to -4.7]), especially for those who were O-negative (O-) (aRR, 0.74 [CI, 0.66 to 0.83]; ARD, -8.2 per 1000 [CI, -10.8 to -5.3]). There was also a lower risk for severe COVID-19 illness or death associated with type O blood group versus all others (aRR, 0.87 [CI, 0.78 to 0.97]; ARD, -0.8 per 1000 [CI, -1.4 to -0.2]) and with Rh- versus Rh-positive (aRR, 0.82 [CI, 0.68 to 0.96]; ARD, -1.1 per 1000 [CI, -2.0 to -0.2]).LimitationPersons who rapidly died of severe COVID-19 illness may not have had SARS-CoV-2 testing.ConclusionThe O and Rh- blood groups may be associated with a slightly lower risk for SARS-CoV-2 infection and severe COVID-19 illness.Primary Funding SourceOntario Academic Health Sciences Centre AFP Innovation Fund and the Ontario Ministry of Health and Long-Term Care.

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