Two new studies have weighed in with fresh evidence of the link between blood type and Covid-19, suggesting that people with blood type O appear to have a lower risk of falling severely ill with the infection while people with blood groups A or AB tend to exhibit greater disease severity. 

The studies, both published in the journal Blood Advances this week, are not the first pieces of research to pick up on this link. A small handful of other studies have also indicated that people with blood type O appear to be less likely than any other blood type to test positive for Covid-19 and less likely to fall seriously sick.

While it’s still not clear whether this link is a direct cause-and-effect relationship or simply a coincidental correlation, the two new bits of research further builds on the idea that blood type might have some role in how Covid-19 affects people. 

The first of the two studies saw scientists look at health registry data from more than 473,000 people in Denmark who tested for Covid-19 and compared it to a control group of more than 2.2 million people from the general population, accounting for almost 40 percent of the total Danish population. Around 42 percent of the Danish population has blood type O and another 42 percent have blood type A. Despite equal representation, fewer people with blood type O caught Covid-19; just 38 percent of the people who tested positive were blood type O, while 44 percent were blood type A. Similarly, people with blood type B and AB also received more positive Covid-19 results than expected.

The second study examined data from 95 critically ill people who were hospitalized with Covid-19 in Vancouver, Canada. They found that patients with blood groups A or AB were more likely to need mechanical ventilation, with 84 percent of A or AB patients requiring mechanical ventilation compared to 61 percent of O or B patients. They also found that blood groups A or AB were more likely to require dialysis for kidney failure, suggesting these blood types might have a higher risk of Covid-19 complications than people with blood types O or B. 

All of these findings are certainly intriguing, but there are a few big caveats that need to be appreciated. First and foremost, both of these studies only found a correlation with no indication of a causal link. This means that it’s not clear how the relationship between blood type and Covid-19 works and any link may be coincidental. It could be, for example, that a certain ethnic group is at a higher risk of catching Covid-19 and they incidentally are likely to share a certain blood type. 

That said, it is known that blood type can influence how your immune system fights against infections in general. For example, people with type A blood produce different antibodies to people with blood type O. It's still not known, however, whether this has any significant effect on a person's ability to battle Covid-19. 

Also importantly, people with blood type A and B should not be worried about these findings, nor should people with blood type O feel they have some superhuman invulnerability to the infection. The overall protective effect of having blood type O is relatively minimal compared to other risk factors such as age, being overweight, and suffering from other health conditions.

As ever, follow your local social distancing measures, keep physically active, and wear a face mask.