WASHINGTON — Evidence has been accumulating that there might be an association between blood type O and a lower risk of COVID-19 and getting severely ill, and now a new study adds to that research. The study, published in the journal Annals of Internal Medicine Monday, found that those with blood type O and negative Rh blood types may have a "slightly lower" risk for coronavirus infection and getting severely ill. The researchers — based in Toronto, Canada — analyzed health data on 225,556 people who were tested for COVID-19 between Jan. 15 and June 30, according to a report in CNN Health. There"s a very small difference, however. The researchers found that 2.9% of those who tested positive for coronavirus had blood type O compared with 4.1% of people with type B, 3.8% of people with type AB and 3% of people with blood type A. When it came to severe illness, the data showed that 0.5% of those with blood type O were among the patients with severe symptoms of COVID-19, or among those who died. That compared with 0.7% of people with blood type B, 0.7% with type AB and 0.6% with type A blood. The study also found that 2.3% of those with rhesus-negative blood type had COVID-19 compared with 3.3% of those with Rh-positive blood type — and 0.5% of those with Rh-negative blood type had severe disease or died compared with 0.6% of those with Rh-positive type. The Rhesus-system is the second most important blood group system after ABO. Yet these findings only suggest an association between blood type and COVID-19 risk. More research is needed to determine the nature of that relationship — and while there are some theories, researchers don"t yet know what mechanism could explain the link between different blood types and COVID-19. — Agencies