A new study published in Blood Advances this week suggests that individuals with Type O blood may be less likely to contract COVID-19 and less likely to experience severe outcomes if they do. The researchers used health registry data from Denmark to analyze the blood types of 473,000 individuals who tested positive for COVID-19 and compared it to the 2.2 million people in the overall population.

Their results suggest that those with Type A, B or AB blood may be at increased risk of contracting the virus and that those with Type A or AB blood may be more likely to become severely ill and need mechanical ventilation. The results imply that those with Type O blood may have some protection against COVID-19 and the potential outcomes with severe illness, including organ damage.

“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,” one of the study’s authors, Dr. Mypinder S. Sekhon from the University of British Columbia told Science Daily. “Of particular importance as we continue to traverse the pandemic, we now have a wide range of survivors who are exiting the acute part of COVID-19, but we need to explore mechanisms by which to risk stratify those with longer-term effects.”

The study isn’t the only one to suggest that blood type may be an indicator of how individuals with COVID-19 will fare — and how likely they are to get it in the first place. A study from the New England Journal of Medicine in June concluded that Type O blood may be associated with a lower risk of contracting COVID-19 and that those with Type A may be more likely to contract it. The authors also mentioned a 2005 study from the Journal of the American Medical Association, which showed a similar connection between Type O blood and the original SARS virus.

Dr. Amesh Adalja, a senior scholar at Johns Hopkins University Center for Health Security, says blood type has been identified as a factor in other illnesses. “We’ve seen in the past — for example, having certain blood types has some influence on norovirus susceptibility,” he says. “But with COVID-19 it seems to be that there are several lines of converging evidence pointing to blood type playing some role. It hasn’t quite been unraveled completely yet, but it looks to me like this is probably a real signal.”

“It’s not ironclad,” adds Adalja. “It doesn't mean that if you have Type O blood that you can live as if there’s not a pandemic ... It just means that this might be something that helps us explain why some people have severe illness and others people do not.”