What Doctors Want You To Know About the Potential Link Between COVID-19 and Blood Type

We’re all painfully aware of the many unanswered questions about COVID-19. The relatively new disease caused by the novel coronavirus still has no cure, and so far efforts to contain it have proved difficult as states continue to reopen and people grow weary of social distancing. One of the biggest questions plaguing experts is why otherwise young and healthy people have gotten really sick and died from COVID-19, while others barely have symptoms at all.

Researchers are studying a number of theories, including genetic mutations that can put people at greater risk of severe infection and whether the viral load, or the amount of virus a person is exposed to, plays a role in how severe a person's symptoms are. But a number of new studies point to another factor that could impact a person's experience with the coronavirus: your blood type.

While these studies are a scientifically important starting point, “it is far too early to use this information to apply risk of COVID-19 to a specific person based on their blood grouping,” says Eric A. Gehrie, MD, an assistant professor of pathology and surgery and the co-director of the patient blood management program at Johns Hopkins Hospital in Baltimore, Maryland.

First of all: What are blood types and how do they work?

Your blood plays a big role in your immune system's ability to defend the body. White blood cells circulating in the bloodstream produce antibodies to attack foreign invaders like germs and viruses. And compounds on the surface of your red blood cells called blood group antigens help recognize said foreign invaders.

Your blood type is determined by the presence (or absence) of those A or B antigens on the surface of your red blood cells, which varies depending on your blood type, says Raymond Comenzo, MD, a hematologist and the director of transfusion services at Tufts Medical Center. The vast majority of people fall into one of four major blood types: A, B, O or AB. The most common is group O, and the least common is group AB. Group A people have A antigens, group B have B antigens, group O has neither A or B antigens, and AB has both A and B antigens.

Your cells also make A or B antibodies against the blood type antigens they don’t have. People with group A blood make anti-B antibodies, and people with group B blood make anti-A antibodies. People with group O blood make both anti-A and anti-B antibodies, while group AB people make no anti-A or anti-B antibodies. If your cells come into contact with cells with a different set of antigens than the ones you already have, your antibodies attack the cells, which are seen as invaders.

What do blood types have to do with COVID-19?

Several recent studies suggest a link between blood type and susceptibility to SARS-CoV-2, the virus that causes COVID-19. The studies, however, don’t “even begin to approach the mechanistic side of how this would work,” says Dr. Gehrie, meaning that while there potentially is a connection between the two, we don't know why or how it would work.

The most compelling study, published this month in the New England Journal of Medicine, scanned the entire genome of more than 4,000 people in Spain and Italy for genetic differences among those who were diagnosed with COVID-19 and those who weren’t. They found a weak but significant link to blood type (which is determined by genetics): People with group A blood were at a slightly higher risk of COVID-19 infection, while those with blood group O blood seemed to be slightly protected against COVID-19.

Another study in the British Journal of Hematology also found that group A COVID-19 patients were at higher risk of hospitalization compared to group O patients. And a few other preliminary studies, including an analysis by the genetics testing company 23andMe, have come to similar conclusions—although these haven’t been peer-reviewed and published in a journal.

“The fact that multiple different research groups are reporting this association probably speaks to some kind of association. But at present, the strength of this association is highly uncertain,” says Dr. Gehrie.

Why would your blood type potentially affect your risk of COVID-19?

This isn’t the first time that researchers have made a connection between blood type and other forms of the coronavirus. A 2005 study following the 2003 outbreak of the SARS-CoV-1 virus (which causes the disease SARS) found that people with type A blood were at higher risk of infection from that coronavirus than people with other blood types.

Jacques Le Pendu, PhD, research director at Nantes University in France who studies viruses, was intrigued by these findings and came up with a blood group hypothesis to explain them. His 2008 study manipulated cells in a lab setting to show that those which produced anti-A antibodies blocked the SARS-CoV-1 virus from ever binding to receptors in cells, so the virus was unable to reproduce. “It’s not a formal proof…but it’s a strong indication,” says Dr. Le Pendu.

In order for a virus to make you sick, it has to bind to cells in your body where it can reproduce. The viruses that caused the 2003 SARS outbreak (SARS-CoV-1) and the current COVID-19 outbreak (SARS-CoV-2) reproduce in epithelial cells in the upper respiratory tract. Epithelial cells also express A or B antigens, so as a virus reproduces in a person it’s infected, Dr. Le Pendu says the copies likely carry the same antigens of that person. In a person with type A blood, for example, the virus will have group A antigens. If a person with type B or type O blood is then exposed to that sick person with type A blood, they may not get sick, because their anti-A antibodies recognize the A antigens and block the virus.

A second theory explains why people with some blood types may be more likely to have severe outcomes from a COVID-19 infection. Having type A or B blood is linked to a greater risk of developing blood clots, and unusual blood clots are a noted complication of COVID-19. “These two concurrent hypotheses could work together. They’re not mutually exclusive,” says Dr. Le Pendu.

The doctors interviewed for this article emphasized that there’s no proof yet for either of these theories—and you can explain pretty much anything you want with cherry-picked data. It’s impossible to know if the link between COVID-19 and blood type is due to genetics or another factor without further research.

“The human body is really complex, with all these interacting features,” cautions Eleanor J. Murray, ScD, an assistant professor of epidemiology at Boston University School of Public Health who’s been studying the COVID-19 outbreak. She notes that certain blood types are more common by race and ethnicity. As race seems to  may be a factor in exposure to or treatment for COVID in the first place, which could potentially better explain these findings.

What does this mean for how we fight the virus?

All experts agree that more research is in order. “We need to understand the association more specifically to determine whether there is valuable knowledge relevant to the development of vaccines or treatments,” says Dr. Comenzo.

Dr. Gehrie says that this research may potentially give us a greater understanding of how the virus infects people and causes symptoms, which could help scientists develop preventative measures and treatments based on a person’s individual risk. “For now, the strength of the association is unknown,” he says.

“The real question is how important [blood type] is in terms of risk or protection from the infection or the disease...The truth will only be known only when we understand how it really works,” says Dr. Le Pendu. His team is working on a paper that they hope to submit for peer review in the next few weeks that he says scientifically proves his hypothesis from 2008.

If his theory pans out, it could work like “an imperfect natural vaccine…[but] not a true protection like a good vaccine would provide,” he says. If blood group could offer some protection against getting sick, ultimately it could slow the virus’s spread at a population level. And less spread would mean lower odds of getting sick, even if you have type A or B blood.

One catch is that people have varying levels of antibodies—and many of us have such low levels that they wouldn’t protect us from the virus. But Dr. Le Pendu is hopeful researchers could find a way to increase antibody levels to increase protection. If you receive a blood transfusion with an incompatible blood type—i.e. you have type A blood and receive type B blood—it spurs the body to massively pump out antibodies that can cause serious problems like kidney failure. This also suggests, though, that it is possible to increase antibody levels; experts would have to figure out how to do so safely and cheaply at scale. “We have nothing that works so far. We’re just starting to see how that [could] be done,” says Dr. Le Pendu.

What are the takeaways for everyday people?

Even if the blood group theory does pan out, you can still get sick no matter your blood type. If you’re type O, for example, you can get sick if you’re infected by another type O person. “Your overall risk [would be] slightly lower, but that doesn’t mean you’re protected,” says Dr. Le Pendu. “It slows down the virus transmission overall, but on the individual level you’re still at almost the same risk as others.”

Bottom line: aAnyone of any blood group can potentially become very sick from COVID-19. Until we have a vaccine or a cure, it’s essential to continue following the current guidelines on wearing a mask, washing your hands frequently, and practicing social distancing. “Blood type is almost irrelevant,” says Dr. Comenzo.

The good news is, the world’s best researchers are working on the problem. “The number of trials for treatments are exploding,” says Dr. Murray. Hopefully we’ll have better treatments—and maybe even a full-fledged vaccine—soon.

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