Scientists don’t know whether the vaccine causes the syndrome, and if so, what the mechanism is. “Everyone’s scratching their heads: Is this a real signal?” says Robert Brodsky, a hematologist at Johns Hopkins University. But vaccine safety officials say they did not take the decision lightly, and that symptoms seen in at least 13 patients, all between ages 20 and 50 and previously healthy, in at least five countries are more frequent than would be expected by chance. The patients, at least seven of whom have died, suffer from widespread blood clots, low platelet counts, and internal bleeding—not typical strokes or blood clots. “It’s a very special picture” of symptoms, says Steinar Madsen, medical director of the Norwegian Medicines Agency. “Our leading hematologist said he had never seen anything quite like it.”

A somewhat similar blood disorder, called immune thrombocytopenia (ITP), has been seen in at least 36 people in the United States who had received the Pfizer and Moderna vaccines against COVID-19, The New York Times recently reported. The U.S. Food and Drug Administration said it was investigating these cases, but also said the syndrome did not appear to be more common in vaccinated people, and immunizations in the United States have continued. But Madsen says the cases seen in Europe in recent weeks are distinct from ITP, which lacks the widespread blood clots seen in the European patients.