This article about COVID-19 was originally published by Bloomberg News.

Research from genetic-testing giant 23andMe Inc. found differences in a gene that influences a person’s blood type can affect a person’s susceptibility to Covid-19.

Scientists have been looking at genetic factors to try to determine why some people who contract the new coronavirus experience no symptoms, while others become gravely ill. In April, 23andMe launched a study that sought to use the millions of profiles in its DNA database to shed light on the role genetics play in the disease.

Preliminary results from more than 750,000 participants suggests type O blood is especially protective against SARS-CoV-2, the virus that causes Covid-19, the company said on Monday. The findings echo other research that has indicated a link between variations in the ABO gene and Covid-19.

Many other groups, including 23andMe competitor Ancestry Inc., are combing the genome to help make sense of the virus. It is known that factors such as age and underlying health conditions can determine how people fare once they’ve contracted Covid-19. But those factors alone don’t explain the wide diversity of symptoms, or why some people contract the disease and others don’t. Studying the genetics of the people who are more susceptible to SARS-CoV-2 could help identify and protect those more at risk, as well as help speed treatment and drug development.

Several other studies looking at both severity of illness and susceptibility to disease have also suggested blood type plays a role.