People who have type O and rhesus negative (Rh-) blood may have a slightly lower risk of infection with SARS-CoV-2 than those with other blood types, according to a recent study. On the opposite end of the spectrum, people with B+ blood were twice as likely to get infected compared to those with O-.

ABO status and COVID-19: what’s the connection?

Researchers at the University of Toronto assessed the prevalence of SARS-CoV-2 infection, as well as the severity of COVID-19 illness or death of 225,556 patients who had their ABO blood group assessed between January 2007 and December 2019, and who were subsequently tested for SARS-CoV-2 between January 15 and June 30, 2020.

After accounting for common comorbidities (cardiac disease, chronic kidney disease, anemia, cancer, asthma, etc.), age, and other relevant characteristics, the researchers compiled an adjusted relative risk for each blood group.

According to the results, the O- blood group had a 2.1% risk of becoming infected with SARS-CoV-2, the lowest unadjusted probability out of all blood groups. The highest unadjusted probability of infection was 4.2% among the B+ blood group. There was also a lower risk for severe COVID-19 illness or death associated with type O blood group versus all others.

No safety in blood types

However, this doesn’t mean that people with O- blood are immune to the coronavirus — it’s just that they may be slightly less likely to get infected. As such, those with O- blood should still get vaccinated against COVID-19 and take all the necessary precautions.

The study also comes with its own limitations. For instance, the sample selected for this study was not random. It included only people who had their blood tested, and this non-randomness in sample selection tends to complicate the analysis and reduce the confidence in the results. The demographics also weren’t representative of the general population, with the mean age being 53.8 years and 29% of the participants being men.

But the sample size itself was adequate, numbering hundreds of thousands of participants, so people with O- blood may indeed have a favorable edge against the coronavirus. This fact is corroborated by other studies, such as research by Danish scientists who compared data from around 473,000 COVID-19–positive individuals with a control group of 2.2 million people in the general population, finding fewer infected people with blood type O and more people with A, B, and AB types.

Many other studies seem to have reported an association between type O blood and a reduced incidence of coronavirus infections. A study by researchers at Massachusetts General Hospital looking at patients in Italy and Spain found that blood type O had a 50% lower risk of severe coronavirus infection (i.e. needing intubation or supplemental oxygen) compared to patients with other blood types.  Another study from April 2020 found that among 1,559 coronavirus patients in New York City, a lower proportion than would be expected had Type O blood. And very early in the pandemic, a study of over 2,100 coronavirus patients in Wuhan and Shenzhen (not peer-reviewed) found that people with Type O blood had a lower risk of infection.

“Studies of the accuracy of serologic tests for anti-SARS-CoV-2 immunoglobulins may assess whether there is variation in antibody titers by ABO and Rh status,” the authors noted in the Annals of Internal Medicine. “Furthermore, among ongoing clinical trials of immunotherapy using convalescent plasma or of SARS-CoV-2 vaccines, the interaction between participant blood groups and therapeutic efficacy could be measured.”