Since blood group distributions vary among ethnic subgroups, the researchers also controlled for ethnicity and maintained that fewer people with blood type O tested positive for the virus.
People with blood type O may have a lower risk of infection from the SARS-CoV-2 virus, that causes COVID-19, and reduced likelihood of severe outcomes, including organ complications, if they develop the disease, two new studies suggest.
The studies, published in the journal Blood Advances on Wednesday, add evidence that there may be an association between blood type and vulnerability to COVID-19.
However, the researchers noted that more studies are needed to better understand why and what it means for patients.
In the first study, researchers compared Danish health registry data from over 473,000 individuals tested for COVID-19 to data from a control group of more than 2.2 million people from the general population.
Among the COVID-19 positive, they found fewer people with blood type O and more people with A, B, and AB types.
The study suggests that people with blood types A, B, or AB may be more likely to be infected with COVID-19 than people with type O.
The researchers did not find any significant difference in rate of infection between A, B, and AB types.
Since blood group distributions vary among ethnic subgroups, the researchers also controlled for ethnicity and maintained that fewer people with blood type O tested positive for the virus.
“It is very important to consider the proper control group because blood type prevalence may vary considerably in different ethnic groups and different countries,” said study author Torben Barington, from the University of Southern Denmark.
A separate retrospective study in Canada found that people with blood groups A or AB appear to exhibit greater COVID-19 disease severity than people with blood groups O or B.
The researchers examined data from 95 critically ill COVID-19 patients hospitalised in Vancouver.
The team found that patients with blood groups A or AB were more likely to require mechanical ventilation, suggesting that they had greater rates of lung injury from COVID-19.
The researchers also found more patients with blood group A and AB required dialysis for kidney failure.
These findings, taken together, suggest that patients in these two blood groups may have an increased risk of organ dysfunction or failure due to COVID-19 than people with blood types O or B.
While people with blood types A and AB did not have longer overall hospital stays than those with types O or B, they did remain in the intensive care unit (ICU) for a longer average time, which may also signal a greater COVID-19 severity level, the researchers said.
