Does having a certain blood type put you at higher risk of developing a severe case of Covid-19? A recent report doesn’t have all the answers, but it does offer some clues.

The report, which analyzes the data of almost 2,000 patients hospitalized for Covid-19 in Italy and Spain, found that those with type A blood were at increased risk for respiratory failure. Type O blood, on the other hand, was found to have a protective effect.

This apparent susceptibility and protection from respiratory failure is far from absolute. Some patients with blood type O still suffered respiratory failure, while some with type A did not. Infection by SARs-CoV-2 has led to respiratory failure in patients of all blood types, including types A, O, B, and AB.

These findings, rather than being definitive, add to what was previously known about the role of blood types in susceptibility to infection by SARS-CoV-2. Groups in China, the United States, and Italy have all found that people with blood type A are over-represented amongst those infected.

These studies all highlight a set of six genes located on the chromosome near the region that encodes the ACE2 protein, the receptor of SARS-CoV-2. Curiously, although many variants of the receptor itself are known, none are yet associated with an increased risk of infection or respiratory failure. The proximity of this region to the ACE2 receptor prompts speculation that it may modify the receptor in some unknown way, influencing both infection by the virus and progression of the disease.

How might we interpret the practical implications of these results? First, they do not tell us that those with blood type O aren’t at risk of contracting, falling ill, or dying from Covid-19. Neither do they tell us that everyone with blood type A who is infected will suffer respiratory failure. As far as we know, everyone—regardless of genetic type—is at risk of infection and disease.

The results do tell us that those who know their blood is type A must continue taking appropriate precautions to protect themselves from Covid-19. It also means healthcare workers should take note of the blood type of their patients and, if a patient is type A, be on high alert for signs of respiratory failure. In the future, it may be wise to test hospitalized Covid-19 patients for variants in and around the ACE2 locus for the same reason.

Inherited predisposition is the latest of many factors associated with respiratory failure due to SARS-CoV-2, including age, heart disease, pulmonary disorders, diabetes, and smoking history. Every day we are learning more about the disease, how it is transmitted, who is most susceptible, and how it affects our bodies. With knowledge comes a deeper understanding of how to prevent transmission, and how to care for those who fall ill. As is the case with the findings around blood types described here, we benefit from the efforts of scientists and doctors working across many countries to understand, control, and treat this disease.