Association Between ABO and Rh Blood Groups and SARS-CoV-2 Infection or Severe COVID-19 Illness : A Population-Based Cohort Study

Ann Intern Med. 2021 Mar;174(3):308-315. doi: 10.7326/M20-4511. Epub 2020 Nov 24.


Background: The ABO and rhesus (Rh) blood groups may influence risk for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

Objective: To determine whether ABO and Rh blood groups are associated with risk for SARS-CoV-2 infection and severe coronavirus disease 2019 (COVID-19) illness.

Design: Population-based cohort study.

Setting: Ontario, Canada.

Patients: All 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.

Measurements: The 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.

Results: A 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]).

Limitation: Persons who rapidly died of severe COVID-19 illness may not have had SARS-CoV-2 testing.

Conclusion: The 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 source: Ontario Academic Health Sciences Centre AFP Innovation Fund and the Ontario Ministry of Health and Long-Term Care.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ABO Blood-Group System*
  • Aged
  • COVID-19 / blood*
  • COVID-19 / diagnosis
  • COVID-19 / mortality
  • COVID-19 Nucleic Acid Testing
  • Female
  • Humans
  • Male
  • Middle Aged
  • Ontario / epidemiology
  • Prognosis
  • Retrospective Studies
  • Rh-Hr Blood-Group System*
  • Risk Factors
  • SARS-CoV-2
  • Severity of Illness Index


  • ABO Blood-Group System
  • Rh-Hr Blood-Group System