En español | In the last several months, a number of studies have drawn a connection between blood type and COVID-19 risk, and most have reached the same conclusion: People with type O blood, the most common kind, may have a slight advantage over their peers when it comes to risk for a coronavirus infection and hospitalization or death from COVID-19. This, however, does not mean they can't contract the virus or fall seriously ill from it.
Researchers in Denmark found that among more than 7,400 people who tested positive for COVID-19, fewer individuals had type O blood compared to type A, despite the fact that the two blood types accounted for the same share of the population when compared to a larger control group. Canadian researchers reached a similar finding in their retrospective study published in Annals of Internal Medicine. They found that people with type O blood had a lower risk for contracting the coronavirus (SARS-CoV-2) compared to those with type A, B or AB. They also observed that individuals with type O blood had a slightly lower risk for getting severely ill or dying from COVID-19 if they did become infected. And several other peer-reviewed studies reinforce these findings.
Even still, experts caution that the accumulating evidence on this subject shouldn't influence everyday medical or public health decisions.
"I would never tell someone who is type O that they don't have to wear a mask, or they don't have to do social distancing, or they don't have to wash their hands frequently,” says Roy Silverstein, M.D., professor and chair of medicine at the Medical College of Wisconsin Division of Hematology and Oncology, who was not involved in the studies. “They are at risk for COVID, just a little bit lower than type A.… That doesn't mean that [type] O is zero risk.”
Most of the value in the research, Silverstein says, is to learn more about the new virus that has so far infected almost 68 million people worldwide, and how the body responds to it.
Blood type may influence other infections, as well
The blood type–infection connection is not unique to the coronavirus. “There's a fairly decent amount of existing literature beyond SARS-CoV-2” that certain blood types can play a role in disease risk and severity, says Joel Ray, M.D., a clinician scientist and professor at St. Michael's Hospital in Toronto and lead author of the study published in Annals of Internal Medicine.
Research has shown that people with type O blood are more likely to get seriously ill from cholera than others who come down with the illness; and type O blood has been shown to offer some protection against severe malaria. Blood type may also influence norovirus susceptibility.
One explanation: Your blood type is based on proteins, called antigens, that cover the surface of your red blood cells. When these antigens come into contact with a pathogen or other unfamiliar substance, they trigger an immune system response. And certain blood types may be better at fighting off certain foreign invaders than others.
More specific to the coronavirus is the theory of clotting. People with type O blood tend to clot less that their peers, Ray says. “Clotting is always a good thing if you cut yourself, but it's a bad thing, generally, if you get a really bad infection [because it] activates the clotting cascade and tends to activate clotting in small blood vessels,” he says. And a complication seen in many people with severe COVID-19 is blood clots in the small blood vessels of the lungs.
"There is a precedent in biology for blood type influencing infection,” Silverstein says. And though researchers still don't know why or how blood type may interact with SARS-CoV-2, a better understanding could “lead to discovering new pathways or new targets for treatment,” he adds.
'Everybody needs to be more careful,’ regardless of blood type
Despite the growing body of research suggesting some link between COVID-19 and blood type, experts say these studies don't change much for the individual. It's unlikely that doctors will make treatment decisions based on blood type, Silverstein says, and he cautions that blood type should not be considered on the same level of risk as age, obesity or other underlying health conditions.
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A June report from the Centers for Disease Control and Prevention (CDC) found that hospitalizations for people with COVID-19 were six times as high for patients with chronic health conditions like heart disease, lung disease and diabetes, compared to otherwise healthy individuals. Deaths among this population were 12 times as high. What's more, 95 percent of COVID-19 deaths in the U.S. have occurred in adults 50 and older.
"If you're type A blood, you might be at a slightly higher risk than if you're type O. But that doesn't mean you should be more careful; everybody needs to be more careful. And that the worst thing that can happen is that people let their guard down,” Silverstein says.
Breakdown of blood types in U.S. population