Over the past year, a number of studies have hinted that people with certain blood types may be slightly more vulnerable to severe cases of COVID-19. This curious link has previously escaped explanation, but new research claims to have solved at least part of the puzzle.

Scientists have found that SARS-CoV-2 appears to have a strong preference for binding to blood group A antigens found on respiratory cells in the lungs. It’s not clear why the virus has this affinity for blood group A antigens, but it could explain why people with blood group A are slightly more likely to fall severely sick with COVID-19. 

"Our observation is not the only mechanism responsible for what we are seeing clinically, but it could explain some of the influence of blood type on COVID-19 infection," Dr Sean R Stowell, MD, PhD, study author from Brigham and Women's Hospital in Boston, said in a statement.

The findings were reported in the journal Blood Advances this week. 

SARS-CoV-2, the virus that causes COVID-19, infects human cells using its receptor-binding domain (RBD) – a key part of the virus located on its “spike” protein that allows it to dock to host cells and enter them. In an in vitro experiment, the researchers looked at how the RBD of SARS-CoV-2 interacted with synthetic versions of blood group antigens found on the respiratory and red blood cells with blood groups A, B, and O.

Simply put, two antigens – A and B –  determine a person's blood group: blood type A has A, blood type B has B, blood type AB has both, and blood type O has neither. These antigens are most often associated with the outside of the red blood cell, but they can be also found in other cells, such as respiratory cells in the lung.

The researchers found that the RBD was significantly more prone to binding to blood group A antigen found on respiratory cells. However, it did not display any particular preference for blood group A antigens on red blood cells, nor the respiratory and blood cells of other blood groups. 

"It is interesting that the viral RBD only really prefers the type of blood group A antigens that are on respiratory cells, which are presumably how the virus is entering most patients and infecting them," explains Stowell. "Blood type is a challenge because it is inherited and not something we can change. But if we can better understand how the virus interacts with blood groups in people, we may be able to find new medicines or methods of prevention."

This is likely to only be part of the story, however. Earlier preliminary research has indicated that a number of other factors may also be at play when looking at why certain blood groups are apparently linked to a higher risk of severe COVID-19 infections.

It’s worth considering that the risk associated with different blood groups is relatively subtle. Although people with blood type O were less likely than any other blood type to test positive for COVID-19, this doesn’t mean they have "superhuman" protection against the disease, nor does it mean people with blood type A are desperately vulnerable. 

According to the CDC, the main risk factors for developing severe COVID-19 are cancer, chronic kidney disease, Down syndrome, chronic obstructive pulmonary disease, heart conditions, obesity, pregnancy, smoking, and Type 2 diabetes mellitus. A range of other underlying conditions may also be linked to an increased risk for severe illness. All things considered, these risk factors are probably the ones to worry about, not your blood type.