Having O and rhesus-negative or Rh-, blood was associated with a slightly lower risk for COVID-19 infection and severe illness or death, say researchers.

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For the results, published in the journal Annals of Internal Medicine, the research team examined 225,556 people who tested their blood type from January 2007 to December 2019, and who later had a COVID-19 lab test. "Our goal was to determine whether ABO and Rh blood groups could be associated with a risk for acquiring a SARS-CoV-2 infection and developing a severe COVID-19 illness," said study authors from the University of Toronto in Canada.

According to the study, out of the more than 200,000 tests that were examined, 1,328 of them were diagnosed with severe COVID-19 illness or death, with higher probabilities among AB and B blood groups, as well as those who were Rh-positive. Their findings indicate that O and Rh-negative blood groups were associated with a slightly lower risk for SARS-CoV-2 infection as well as severe COVID-19 illness or death. "Although we saw a statistically significant association between blood group and severe disease or death, it was also assumed that we had correctly identified severe illness associated with COVID-19 ," the authors wrote.

The research team now wonder whether similar differences in blood type will be seen within ongoing clinical trials studying the therapeutic efficacy of SARS-CoV-2 immunotherapy or vaccination. "At most, a small proportion of SARS-CoV-2 infection or related illness in the entire population could be prevented by some undetermined property conferred by O blood type and, perhaps, further enhanced by Rh- status," the researchers said. "Whether this information can influence COVID-19 prevention or treatment strategies remains to be determined," they noted. Earlier, another study published last month in the journal Blood Advances showed that blood group O is associated with a decreased risk for contracting SARS-CoV-2 infection.