The antibodies resulted in a condition called thrombotic thrombocytopenia, which caused both clotting and abnormal bleeding. The researchers suggested calling the newly identified version in these patients "vaccine-induced immunothrombotic thrombocytopenia" or VITT.
Scientists have put forward various theories of what triggers the immune response. The AstraZeneca vaccine uses a chimpanzee adenovirus to transport DNA into recipients and trigger an immune response against the coronavirus. Laboratory research has shown that the chimpanzee virus or DNA could be causing the problem. Some researchers have suggested that bleeding from the injection mixed with the vaccine could bring platelets into the crosshairs of the immune system.
Dr. Greinacher called the theories plausible but unproven.
The article described special blood tests that could help diagnose the disorder and differentiate it from other, more common, clotting problems unrelated to the vaccine. The research team suggested treatment with a blood product called intravenous immunoglobulin, which is used to treat various immune disorders. Dr. Greinacher compared the treatment to putting out a fire.
Medicines called anticoagulants or blood thinners can also be given. However, the researchers advised against prescribing a commonly used heparin because the vaccine-induced condition is very similar to a severe reaction that is rarely seen in people given heparin.
The second report from Norway described five patients, one male and four female health workers aged 32 to 54, who had blood clots and bleeding seven to ten days after receiving the AstraZeneca vaccine. Four had severe blood clots in the brain and three died. Severe headache was one of her early symptoms. Like the German patients, they all had high levels of antibodies that could activate blood platelets.
The team from Norway also recommended intravenous immunoglobulin treatment. The researchers said the disorder is rare but "a new phenomenon with devastating effects on otherwise healthy young adults," and they suggested that it could be more common than previous studies with the AstraZeneca vaccine had shown.
On Friday, European regulators also said they were reviewing reports of some blood clot cases that have occurred in people who had received the Johnson and Johnson vaccine. In the United States, federal agencies are investigating reports of another type of unusual blood disorder in which a few dozen people who received either the Pfizer-BioNTech or Moderna vaccines experienced steep decreases in platelet counts.
Benjamin Mueller and Melissa Eddy contributed to this.