Early research suggested that individuals with Type A blood were at especially high risk of severe complications from Covid-19—but newer research is revealing a more complicated picture, the New York Times' Carl Zimmer reports.

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Researchers link blood type to Covid-19 risk

Earlier this year, scientists released research indicating that individuals' blood type could linked with their risk of contracting the novel coronavirus and developing a severe case of Covid-19, the disease caused by the virus.

One preprint study released last month, which hasn't been peer-reviewed, 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. Specifically, the researchers found that having blood type A was linked with a 50% increase in the likelihood a patient would develop a severe case of Covid-19.

A separate preprint study conducted by researchers in China that hasn't been peer-reviewed found similar results. The study found that, out of 2,173 Covid-19 patients, blood type A was associated with a higher risk of death from Covid-19 as well as a higher risk of contracting the new coronavirus. Patients with blood type O appeared to be the least likely to contract the virus.

Despite the findings, Andre Franke—a molecular geneticist at the University of Kiel in Germany, who led the first study—last month said researchers were still unsure exactly how a person's blood type plays a part in how Covid-19 affects them. "That is haunting me, quite honestly," he said.

Still, some observers hoped the findings could help scientists identify treatments for Covid-19, though they said more research on the topic was needed.

New research raises questions about blood type's role in Covid-19 risk

However, two new studies suggest the link between Covid-19 a person's blood type might not be significant enough to actually alter a person's risk.

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, Zimmer reports. However, Tatonetti and Zietz cautioned that their results related to the AB blood type are less reliable than their results for other blood types, because there were few patients in the study with type AB blood.

Tatonetti and Zietz haven't yet published the results from their full study, which is currently under review for possible publication in a scientific journal, Zimmer reports. The researchers have released preliminary results, which are based on a sample of 1,559, in a preprint study that hasn't been peer-reviewed.

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 new 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.

So what does blood type have to do with Covid-19 risk?

Overall, Tatonetti said connections between individuals' blood type and their Covid-19 risk are 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.

Anahita Dua, a vascular surgeon at MGH and senior author of the study by MGH researchers, said the link they 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.

Joern Bullerdiek, director of the Institute for Medical Genetics at University Medicine Rostock, said the new findings mean "it's probably decided that blood groups are not influencing the outcome of" Covid-19.

But researchers said even though the associations between blood type and the novel coronavirus aren't particularly strong, they could help scientists understand how the virus and Covid-19 work.

According to Zimmer, different blood types make different antibodies, meaning a person's blood type could determine how a patient's immune system battles infections. So, "It's conceivable that these molecular differences in the immune system explain the purported link between blood type and coronavirus infections," Zimmer reports.

Zietz told Zimmer that some research also indicates that "certain blood types have different risks of clotting," which could affect how patients experience Covid-19, which has been shown to cause blood clots in some patients.

Tatonetti said the connection between blood type and clotting risk is "ripe for experimentation," adding, "There are so many people getting infected now across the country that there will be an opportunity to study a much wider population" (Zimmer, New York Times, 7/15).