The COVID-19 pandemic has been the largest global health crisis of the twenty-first century. Doctors and scientists around the world have spent hours researching the virus in order to better understand how it spreads, how it attacks the body, and how to prevent and cure it.

One aspect of the novel coronavirus that has puzzled doctors and scientists since the outbreak began is how for some, the virus produces only mild symptoms, while others require supplemental oxygen and ventilators in order to recover.

In March, preliminary research from Wuhan suggested that blood type could indicate how susceptible an individual is to a severe case of COVID-19. Since then, a significant amount of research has been done to investigate this claim and the results are in: People with blood type O are more protected against the novel coronavirus.

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Type O Blood and COVID-19

Studies have now been conducted in Wuhan, Italy, and Spain, as well as by the personal genetics company 23 and Me. Each of them has come to the same conclusion: people with O-type blood seem to have a lower risk of developing a severe case of COVID-19 if they get the virus at all [1].

The study from 23andMe assessed more than 750 thousand people and found that people with the O blood type are nine to eighteen percent less likely to test positive for COVID-19 than individuals with type A or B [1].

The study from Spain included nearly two thousand participants and concluded that there was a higher risk of contracting COVID-19 for A-positive individuals, while blood type O appeared to have a protective effect against the virus [2].

Another study published in the New England Journal of Medicine found that people with blood type A have a fifty percent greater risk of requiring oxygen support or a ventilator if they become infected with COVID-19, while people with O-type blood appear to have a fifty percent reduced risk of of severe infection [3].

What is Blood Type?

Blood is classified into two groups based on whether or not they have certain molecules (called antigens) on the surface of their red blood cells. These antigens can either be proteins or complexes of sugar molecules [4].

Antigens have a number of roles in the blood, one of which is recognizing foreign cells in the bloodstream. When this happens, the body is prompted to mount an immune response. Some blood types are associated with more severe immune reactions than others.

Two of the main antigens used for blood typing are the “A antigen” and the “B antigen”. People with A-type blood only have A antigens on their red blood cells, people with B-type blood only have B antigens, people with AB-type blood have both, and people with O-type blood have neither [5].

AB-type blood is the rarest blood type, while O-type blood is the most common [5]. 

Read: You Should Know These 10 Little-Known Facts About Your Blood Type

Finding Answers in Our Genes

The research from the New England Journal of Medicine was what’s called a genome-wide association (GWAS) study. It determined gene variants in two regions of the human genome that correspond to a greater risk of severe COVID-19 and COVID-19 related death [3].

One of these regions plays various roles in the immune system, while the other determines your blood type [3].

Still, the answer as to why blood type seems to have an effect on an individual’s risk for severe COVID-19 is not exactly known. Andre Franke, the lead researcher for the study conducted in Spain and Italy, says that the region that codes for blood type is associated with higher levels of key immune molecules, which could be why there is an association [1].

Specifically, people with A-type blood develop antibodies for B antigens, and people with B-type blood develop antibodies for A antigens. Those with type O blood have antibodies for both [1]. 

The theory is that people with O-type blood may have some anti-A antibodies, which could protect them from the virus. That being said, people with B-type blood, who also have anti-A antigens, do not seem to be protected, which means there must be something else about blood type O that gives it a protective effect [6].

Another possible answer has to do with blood clotting. Type O blood has lower levels of proteins that promote clotting, while type A seems to be making antibodies that promote clotting. This could be one reason behind some of the thrombotic events (heart attacks, strokes, and serious breathing problems) that have been seen in many COVID-19 patients [6].

This, however, does not explain why people with type O blood seem to have a lower risk of infection, but rather why they may have a lower risk for severe infection. 

Jacques Le Pendu, a glycobiologist at the University of Nantes, explains that the novel coronavirus can replicate in cells that that express blood type antigens, so when an infected individual coughs, they could release virus particles that are coated in their blood type antigens. People with type O blood, then, will have antibodies against the virus from someone with type A blood, but a person with type A blood will not have the same antibodies [1].

Genes Aren’t the Only Answer

While this research is very intriguing, and is providing some insight into individual virus susceptibility, much more research needs to be done. 

There are many other factors that contribute to an individual’s risk for contracting COVID-19, and just because you have type O blood does not mean that you are completely safe from the virus. Underlying health conditions like heart disease, chronic lung disease, and diabetes increase your chance of getting very sick or dying from COVID-19, and likely have a greater impact on individual outcomes [5].

That being said, Francis Collins, director of the National Institutes of Health, says that a genetic test and a person’s blood type could be useful in identifying those who might be at a greater risk for developing serious illness. 

“The hope is that these and other findings yet to come will point the way to a more thorough understanding of the biology of COVID-19,” [5].

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