Early in the Covid-19 pandemic, researchers found preliminary evidence suggesting that people’s blood type might be an important risk factor — both for being infected by the virus and for falling dangerously ill.
But over the past few months, after looking at thousands of additional patients with Covid-19, scientists are reporting a much weaker link to blood type.
Two studies — one at the Massachusetts General Hospital and the other at Columbia Presbyterian Hospital in New York — did not find that Type A blood increases the odds that people will be infected with Covid-19.
The new reports do find evidence that people with Type O blood may be slightly less likely to be infected. But the effect is so small that people shouldn’t count on it. “No one should think they’re protected,” said Nicholas Tatonetti, a data scientist at Columbia University.
Reviewing medical records for 7,770 people who tested positive for the coronavirus, Dr. Tatonetti and a graduate student, Michael Zietz, found that people with Type A blood were at a somewhat lower risk of being placed on ventilators. People who were Type AB were at a higher risk, but the scientists cautioned that this result might not be reliable because there were so few patients with that blood type in their analysis.
Dr. Tatonetti and Mr. Zietz released the initial results from 1,559 patients at Columbia Presbyterian Hospital in April. Their larger survey is now under review for publication in a scientific journal.
The other new study, carried out at Massachusetts General Hospital, offers a somewhat different picture. The researchers also found that people with Type O were slightly less likely to get Covid-19. But blood type did not affect whether people would have to be placed on ventilators, or their odds of dying.
Anahita Dua, a vascular surgeon at the hospital and the senior author of the study, said that blood type was not something she’d consider when judging the risks faced by patients who tested positive for Covid-19. “I wouldn’t even bring it up,” she said.
“With this new paper, it’s probably decided that blood groups are not influencing the outcome of the disease,” said Joern Bullerdiek, the director of the Institute for Medical Genetics at University Medicine Rostock in Germany.
Even if blood types don’t matter much for treating people with Covid-19, they could reveal something important about the basic nature of the disease.
That’s because blood type influences how your immune system fights against infections. People with Type A blood don’t make the same kind of antibodies as people with Type B blood, for example. It’s conceivable that these molecular differences in the immune system explain the purported link between blood type and coronavirus infections.
But Mr. Zietz saw other possibilities. “There’s some evidence that certain blood types have different risks of clotting,” he said.
Doctors initially treated Covid-19 as a flu-like respiratory disease that mainly damaged the lining of the lungs. But it’s become clear that the coronavirus can cause vast numbers of tiny blood clots.
“That’s a connection that’s ripe for experimentation now,” Dr. Tatonetti said.
Although Dr. Tatonetti may now have a hard time finding many patients to study the blood type link in New York, others can certainly take up the challenge.
“There are so many people getting infected now across the country that there will be an opportunity to study a much wider population than just New York City,” he said.