Blood Type A Linked to Severe COVID-19, But Science Remains Inconclusive

When the pandemic was still new, Chinese researchers published preliminary findings suggesting that people with blood type A faced higher infection risk from the coronavirus, while those with type O seemed protected. The observation sparked dozens of investigations worldwide, each trying to answer a deceptively simple question: Does the virus care what blood type you are?

The question matters because blood type has been shown to influence vulnerability to other diseases—malaria, hepatitis B, HIV, and bacterial infections have all demonstrated associations with ABO groups. When researchers at Harvard and Emory examined the coronavirus at the molecular level, they found something striking. The virus showed what they called a "strong preference" for binding to proteins present only on type A blood cells, particularly those lining the lungs. The study, published in Blood Advances in March 2021, offered what appeared to be direct evidence of a biological mechanism linking blood type to disease risk.

But the scientific picture fractured almost immediately. A Boston hospital study published months earlier found that types B and AB carried higher infection risk, while type O showed protection—yet type A appeared statistically neutral. A Danish study of over two million people confirmed type O's protective effect against infection but found blood type irrelevant to hospitalization or death. Other investigations produced still different results. Scientists interviewed by the BBC cautioned that these preliminary findings should not trigger alarm or carelessness with preventive measures. The variation, they explained, reflected the messy reality of how science actually works: researchers using different methods, studying different populations, wrestling with confounding factors that blur the picture.

In Brazil, researchers stumbled onto their own evidence almost by accident. Gil de Santis, a blood specialist at a São Paulo teaching hospital, noticed something odd while running a clinical trial on convalescent plasma treatment. His team had to type patients' blood before transfusing them, and he kept seeing far more type A patients among the severely ill than the local population would predict. When his group compared 72 severe COVID patients against 160 controls from the same region, type A appeared in 51 percent of the sick versus 30 percent of the healthy. Type O showed the inverse pattern. The math suggested type A carried 2.5 times greater risk of severe disease. Yet Santis was careful to contextualize the finding: coronary disease increased severity risk twentyfold, diabetes five to sevenfold. Blood type, he emphasized, was a minor player in a much larger cast.

Other Brazilian teams reported similar patterns. In Rio Grande do Sul, a blood bank noticed they were transfusing more type A blood to critically ill COVID patients than their usual demand would suggest—47 percent versus their normal 35 percent. In Goiás, however, researchers analyzing plasma donors found type O most common among those who had recovered, with type A second, matching the general population distribution. When they looked at severe cases, they found type AB predominant and type A nearly absent—contradicting the other studies entirely. The hematologist leading that work, Maria Amorelli, noted that the sample was small and donor populations skew toward type O because of a widespread belief that it is the universal donor. She advised against panic: many type A patients had mild disease, many type O patients had severe cases.

The biological mechanisms proposed to explain any connection remain speculative. Type A blood carries the A antigen, a protein on cell surfaces that the virus appears to favor. But blood type also determines which natural antibodies a person carries—type A individuals have anti-B antibodies, type O individuals have both anti-A and anti-B. Some researchers hypothesize these antibodies might offer protection against viral entry. A third factor emerged in the research: type A blood tends toward greater clotting activity, and severe COVID frequently triggers dangerous blood clots. Type O blood shows less clotting tendency, potentially offering another protective mechanism.

Sean Stowell, one of the Harvard researchers, acknowledged in correspondence with the BBC that the ideal study to settle the question would be impossible to conduct ethically. A truly definitive experiment would require deliberately exposing people with different blood types to equal amounts of virus—something no review board would ever approve. Researchers are therefore left with correlational studies and laboratory experiments, each with inherent limitations. New variants of the virus, Stowell noted, would require new investigation. The science has not reached consensus, and may never reach certainty. For now, having type A blood is not a reason for despair, and having type O is not a reason for complacency.