One of the goals of COVID-19 research is understanding why some people develop mild or moderate cases while others experience life-threatening illness. Researchers have made progress in understanding some of the factors that make a difference, including obesity and underlying health conditions like diabetes and heart disease.

Recently, an ongoing study by European scientists has suggested one more potential factor to consider: blood type. Preliminary results of this investigation (which have not yet been peer reviewed) were shared on June 2 on the preprint service MedRxiv.

The researchers scrutinized blood samples from 1,610 hospitalized patients in Italy and Spain with the disease, as well as a 2,205 healthy people in a control group. Their analysis identified variations at two distinct areas on the genome (the complete set of human DNA, including all genes) that were associated with greater risk for severe reactions toSARS-CoV-2, the virus that causes COVID-19, including respiratory failure.

One of these areas on the genome is related to blood types. The researchers found that type A blood was associated with a 50 percent increase in risk that a patient would become extremely ill with COVID-19 and need supplemental oxygen or a ventilator.

This conclusion supports findings from early research done in China, which appeared March 27 in MedRxiv. This gave the researchers more confidence in the associations, says study coauthor Andre Franke, PhD, professor of molecular medicine at the Institute of Clinical Molecular Biology at University Hospital Schleswig-Holstein in Kiel, Germany.

Researchers on another ongoing study, by genetic testing firm 23andMe, released preliminary data on June 8 suggesting that type O blood is protective against COVID-19. The researchers found that people with type O blood are between 9 and 18 percent less likely to test positive for COVID-19 than other blood types.

The 23andMe study is still recruiting subjects, but already has 750,000 participants and is likely to come out with more data regarding genetic associations and COVID-19.

RELATED: The New Normal: What We Know About the Coronavirus So Far and How We Got Here

Dr. Franke hopes to build on the findings about type A blood withmore targeted research, he says, especially because there are 36 known human blood groups. In addition to the four main types A, B, AB, and O there is also a deeper classification system that includes different combinations of antigens (molecules on the surface of every red blood cell) and other substances.

There are other types of diseases where blood types and blood groups affect a persons susceptibility. For example, people who lack a specific type of antigen, called a Duffy antigen, have a higher resistance to malaria.

In terms of why a variant like the gene related to blood type would have significance for COVID-19, Franke says there are three possible hypotheses.

One is that the genetic variant itself contributes to the so-called cytokine storm, in which a persons immune system goes into overdrive in response to the novel coronavirus threat, releasing large amounts of pro-inflammatory substances called cytokines. An excess of cytokines can damage healthy tissues.

The second hypothesis is that the genetic variant causes more coagulation (blood clotting) in response to the coronavirus an already observed result of disease progression.

The third theory is that both of these reactions are occurring simultaneously.

There may be other issues at play here, but given the way we know COVID-19 works, these seem the most likely reasons, Franke says. Next steps are to dig deeper into the blood groups system and see if we can pinpoint actual disease causes.

RELATED: What People With Heart Disease Need to Know About COVID-19

What should you do in response to these studies, considering you can't change your blood type? Nothing yet. Theres no need to get to a doctor if youre type A, and on the flip side, theres no reason to relax your precautions against coronavirus transmission like social distancing and hand-washing if youre type O.

Keep in mind these are preliminary results and more research needs to be done to understand how genetic variations truly affect COVID-19.

More than anything, its a nod toward how much we have left to learn about the way this virus operates, and how genetic variants may affect why some people end up in the ICU and others have milder symptoms or even none at all, says Priya Duggal, PhD, director of the genetic epidemiology program at Johns Hopkins Bloomberg School of Public Health in Baltimore.

If we can find genes that may explain some of the risk or protection from this infection, it will give us insight into the mechanism of disease, she says.

If genetics studies help us better understand how COVID-19 affects the body, they may eventually help lead to treatments.

This genetic study is hopefully the first of many that will help us to elucidate disease mechanism, susceptibility to infection, and maybe even antibody response, says Dr. Duggal. That could provide potential targets for therapeutics. We have a lot to learn from this point, but were gaining more insight with every study.

Read more:

Blood Type and COVID-19 Risk - Everyday Health