Photo/Illutration Research data shows COVID-19 patients with type AB blood have a higher risk of becoming seriously ill. (Asahi Shimbun file photo)

COVID-19 patients under age 65 with type AB blood have a higher risk of becoming seriously ill from the virus while those with type O have a lower risk, a joint research coronavirus task force announced on May 17. 

But the causes of why the risks are higher or lower for people with different blood types are still unclear.

The task force made the discovery through a large-scale genomic analysis conducted for Japanese people.

The task force, consisting of experts on infectious diseases, genomic medicine and genetic statistics, was established by Takanori Kanai, a professor of Keio University, and other scholars in May 2020.

In the study, it compared the blood types of two groups.

The first group consisted of 440 COVID-19 patients under 65 with serious cases of the virus, while the second consisted of 2,377 people in the same age group whose blood samples were taken before the outbreak.

The ratio of patients with type AB blood who had serious cases of COVID-19 was found to be 1.4 times higher than that for people with AB blood in the second group.

The ratio of patients with serious cases of the virus who had type O blood was 0.8 times higher than that for people with O blood in the second group.

The results therefore showed people with type AB blood had a 1.6 times higher risk of having serious symptoms of COVID-19 than people with type O blood, which a statistics expert said accounts for a meaningful difference.

The results showed no differences in the ratios of people with type A and type B blood in both groups.

There have been reports overseas that COVID-19 patients with type O blood had a lower risk of having a serious case of the virus, but results of such a study were inconclusive as few Western people have type AB blood.

Other reports have denied there is a relationship between blood types and a person's risk of becoming seriously ill due to a novel coronavirus infection.