The Link Between Blood Type and COVID-19 Risk

Quite a bit of research has been published about blood type and COVID-19 risk. Here, experts weigh in on what you really need to know.

Differences in how people have reacted to COVID-19—and why—have occupied researchers throughout the pandemic. Some people have been re-infected multiple times with the virus, while others appear to have avoided the virus entirely. And while some people develop more severe forms of COVID-19, others develop mild or no symptoms.

Quite a bit of research has been published on the topic of blood type and its role in determining COVID-19 risk. As a result, our understanding of the connection to blood type continues to evolve.

What Is Blood Type?

Blood types are split up into four major groups, all dependent on the presence or absence of two specific antigens on the surface of the blood: A and B, according to the American Red Cross. A protein called the Rh factor can also be present (+) or absent (-) from the blood. Those two factors make up the eight most common blood types: A+, A-, B+, B-, O+, O-, AB+, and AB-.

While it's always helpful to know your blood type, most people won't necessarily need to know that information. In fact, fewer than 57% of all Americans know their blood type, according to a 2019 survey from Quest Diagnostics. Knowledge of your blood type is usually important if you're undergoing a blood transfusion or organ transplant—but in those situations, your medical team will test your blood type beforehand. It's also helpful to know your blood type—specifically the Rh factor–during pregnancy, so your doctor can troubleshoot if the unborn baby has a different Rh factor.

What Research Says About the Link Between COVID-19 and Blood Type

There have been multiple studies on blood types and COVID-19 risk. Some research findings have suggested that people with blood types A and AB are more susceptible to contracting COVID-19, while those with blood type O are less likely to test positive for the virus.

Early in the pandemic, two studies published in the Blood Advances journal in October 2020 showed a possible link between blood type and vulnerability to COVID-19. The first study, conducted by Danish researchers, analyzed data from more than 473,000 people tested for COVID-19 with data from a control group of more than 2.2m non-tested people. It was through these findings, that the Danish researchers suggested: "That blood group O is significantly associated with reduced susceptibility to SARS-CoV-2 infection."

The second study (also from October 2020) from researchers in Canada looked at data from 95 patients who were severely ill with COVID-19. The study found that patients with blood types A and AB were found to be more likely to require mechanical ventilation and to require dialysis for kidney failure.

Since those studies, a fair amount of research has been conducted on blood type and COVID-19 risk. However, the results have been inconsistent and so the connection between COVID-19 and different blood types still isn't clear.

One study published in the Journal of the American Medical Association (JAMA) in April of 2021, reviewed nearly 108,000 COVID-19 cases. Researchers found no link between blood type and COVID-19 risk, adding that more research was needed on the topic.

A comprehensive review of studies published in Seminars in Vascular Surgery in September of 2021 examined peer-reviewed journal articles published from March 2020 to January 2021. Researchers found inconsistent findings but did identify certain trends suggesting that blood type A might predispose one to increased susceptibility of infection with SARS-CoV-2 (the virus that causes COVID-19), or that type O and Rh-negative blood groups might be protective. Still, the study authors wrote, "the impact of blood type on clinical outcomes remains unclear. At this point in time, there does not appear to be any relationship between blood type and COVID-19–related severity of illness or mortality."

The Role of Blood Type and Virus Risk

While the link between blood type and COVID-19 risk is still unclear, it's important to note that there have been links between blood type and diseases in the past. Blood type has been identified as a risk factor in many diseases, from cancer to venous and arterial thromboembolism.

"People with type O blood may be more susceptible to norovirus," said infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security. And people with some blood disorders like sickle cell disease are resistant to malaria, added Dr. Adalja.

But, overall, the link between blood type, genes, and infection risk is a growing area of research. "We all recognize that we're not the same, but we have not been able, on a genetic basis, very often, to determine whether certain people with certain genes are more or less susceptible to get an infection if they're exposed to a germ," said William Schaffner, MD, an infectious disease specialist, and professor at the Vanderbilt University School of Medicine.

There are some theories on why there could be a link: Your red blood cells are covered with molecules that are known as antigens, Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo, told Health. These antigens help prompt a response from your body's immune system.

It could be that antigens for people with type O blood block the spike protein in SARS-CoV-2, and keeps it from entering into your cells, said Dr. Russo. Blood types can also serve as receptors for viruses and bacteria, and that could be another factor or, explained Dr. Russo, there may be some other, completely different component of type O blood that works to prevent infection.

But again, experts say it's too early to know for sure. "We still do not understand all the factors at play," said Anupama Nehra, MD, an assistant professor at the Rutgers New Jersey Medical School and clinical director of hematology oncology at Rutgers Cancer Institute at University Hospital.

What the Blood Link Research Means for You

"There is no real benefit for the individual person," said Torben Barington, DMSc, a clinical professor of immunology at the University of Southern Denmark and co-author of the early Danish study. "All may acquire COVID-19 and all should take the recommended precautions to reduce the risk."

Regardless of if a specific blood type is associated with a greater risk of contracting COVID-19 and developing severe disease, keep in mind that many other factors, such as age or existing health conditions, are likely to play a larger, dominant role in determining personal risk from COVID-19. The Centers for Disease Control and Prevention (CDC) lists risk factors that have been identified for developing severe COVID-19, and blood type does not appear.

With more research, we may one day have a clearer understanding of the connection between COVID-19 and different blood types. "At the end of the day, we're still not sure if blood type makes a difference," said Dr. Russo. "As an individual, you have your blood type—there's nothing you can do about it."

As a whole, experts recommend that people—regardless of blood type—keep following CDC guidelines to prevent the spread of COVID-19. That means getting vaccinated and boosted, social distancing, wearing a mask in public, and washing your hands regularly, among other things.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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