New research adds to the growing body of evidence that people with blood type 'O' may have a lower risk of Covid-19 infection and reduced likelihood of severe outcomes, including organ complications if they do get sick.
Two studies published in the journal Blood Advances suggest that individuals with blood type 'O' may be less vulnerable to Covid-19 infection.
"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.
"We have the advantage of a strong control group - Denmark is a small, ethnically homogenous country with a public health system and a central registry for lab data - so our control is population-based, giving our findings a strong foundation," Barington added.
For the findings, the research team compared Danish health registry data from more than 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 results suggest 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 the rate of infection between 'A', 'B', and 'AB' types.
The findings also showed that blood groups 'A' and 'AB' associated with an increased risk of severe clinical outcomes of Covid-19 infection.
People with blood groups 'A' or 'AB' appear to exhibit greater Covid-19 disease severity than people with blood groups 'O' or 'B', according to a separate retrospective study.
Researchers 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.
They also found more patients with blood group 'A' and 'AB' required dialysis for kidney failure.
"The unique part of our study is our focus on the severity effect of blood type on Covid-19. We observed this lung and kidney damage, and in future studies, we will want to tease out the effect of blood group and Covid-19 on other vital organs," the authors wrote.
Earlier, another study published in the journal Clinical Infectious Diseases that people with blood group 'A' may be more prone to Covid-19 infection while those with blood type 'O' have a lower risk of contracting the virus.