Two studies published Wednesday in the journal Blood Advances add to a growing body of evidence showing there may be a correlation between a person's blood type and their risk of developing a severe case of Covid-19—and suggest one blood type in particular may be tied to a lower risk.

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Previous evidence offered complicated view of correlation between blood type and Covid-19 risk

Early in the novel coronavirus pandemic, research suggested that individuals with Type A blood were at an especially high risk of developing a severe case of Covid-19 or dying from the disease.

For example, one preprint study released in June examined blood samples from 1,610 Covid-19 patients who developed severe cases of Covid-19, which the researchers classified as needing oxygen or a ventilator as part of their treatment. The researchers found that many of the patients who had severe cases of Covid-19 possessed the same variant on a gene that determines a person's blood type, and that having blood type A was linked with a 50% increase in the likelihood a patient would develop a severe case of Covid-19.

In addition, another preprint study conducted by researchers in China found that, out of 2,173 Covid-19 patients, blood type A was associated with a higher risk of death from Covid-19 and a higher risk of contracting the new coronavirus. Those researchers also noted that patients with blood type O appeared to be the least likely to contract the virus.

However, later research revealed a more complicated picture, suggesting that the link between Covid-19 a person's blood type might not be significant enough to actually alter a person's risk.

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For instance, after reviewing the medical records of 7,770 people who tested positive for the novel coronavirus, Nicholas Tatonetti, a data scientist at Columbia University, and graduate student Michael Zietz said they found that having blood Type A blood was associated with a lower risk of being placed on a ventilator, while having blood type AB was associated with a higher risk of needing ventilation. But overall, Tatonetti said connections between individuals' blood type and their Covid-19 risk were not strong enough to consider blood type as a risk factor for contracting the new coronavirus or developing a severe case of Covid-19.

"No one should think they're protected" because of their blood type, he said.

Separately, researchers at Massachusetts General Hospital (MGH) in a study published in Annals of Hematology, which has open access options, found that people with Type O blood appeared to have a slightly lower risk of contacting the novel coronavirus—but they also found that a patient's blood type was not associated with their risk of needing ventilation or dying because of Covid-19. Anahita Dua, a vascular surgeon at MGH and senior author of the study, said the link she and her colleagues found between blood type and coronavirus risk was so weak that she "wouldn't even" consider using blood type as a risk factor for the virus or Covid-19.

New research sheds more light on the connection

But now, two new studies offer more evidence suggested there is, in fact, a correlation between a person's blood type and their Covid-19 risk—and that people with blood Type O are less susceptible to the coronavirus overall.

For one study, researchers in Denmark analyzed data on 473,654 people who were tested for the new coronavirus between February and July. In total, 7,422 of those people tested positive for the virus.

The researchers found that 38.4% of those who tested positive for the coronavirus had blood Type O—a finding that seemed low when considering that 41.7% of the untested Danish population had that blood type. In comparison, 44.4% of those who tested positive for the virus had blood Type A, while people with that blood type comprised 42.4% of the untested population.

The researchers wrote that their findings demonstrate that blood Type O "is significantly associated with reduced susceptibility to" the novel coronavirus—though they also noted that their study had several limitations and called for further research on the topic.

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For the second study, researchers in Canada analyzed data on 95 patients in Vancouver who tested positive for the coronavirus between February and April. All of the patients were hospitalized for Covid-19 in an ICU.

Among those patients, the researchers found that those with Type A or AB blood had a median ICU stay of 13.5 days, compared to with a median ICU stay of nine days among patients with blood Type O or B. The researchers also found that the patients with Type A or AB blood were more likely to require mechanical ventilation, at 84%, than patients with blood Type O or B, at 61%.

The researchers wrote that, overall, their findings "demonstrate that critically ill Covid-19 patients with blood [Type] A or AB are associated with an increased risk for requiring mechanical ventilation … and prolonged ICU length of stay compared with patients with blood [Type] O or B." However, they also noted that their research had several limitations and called for more studies on the topic. "Further research is required to delineate the biological mechanisms underpinning these findings," they wrote.

What do the findings mean?

While experts generally agreed that the new findings are interesting, they cautioned that the results are correlational—not causational.

Roy Silverstein, chair of medicine at the Medical College of Wisconsin, called the new studies' findings "interesting cocktail party conversation," and he added that, with further study, they "could lead to new approaches for prevention or therapy." However, he said, "at the present time, there is no reason to think that if you have type O blood, you're protected from Covid-19."

Similarly, Mypinder Sekhon, an intensive care physician at Vancouver General Hospital and an author of the Canadian study, said, "As a clinician … [blood type] is at the back of my mind when I look at patients and stratify them. But in terms of a definitive marker we need repeated findings across many jurisdictions that show the same thing."

Sekhon added that he doesn't believe blood type "supersedes other risk factors of severity" for Covid-19, such as a person's age or comorbidities.

"If one is blood group A, you don't need to start panicking," Sekhon said. "And if you're blood group O, you're not free to go to the pubs and bars" (Edwards, NBC News, 10/14; Fox8, 10/14; Hunt/Howard, CNN, 10/14; Bogetofte Barnkob et al., Blood Advances, 10/14; Hoiland, Blood Advances, 10/14).