Blood Clots Linked to AZ Vaccine Stem From Rare Antibody Reaction

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The antibodies led to a condition called thrombotic thrombocytopenia, which caused both clotting and abnormal bleeding. The researchers suggested naming the newly identified version in these patients “vaccine-induced immune thrombotic thrombocytopenia,” or VITT.

Various theories have been offered by scientists as to what touches off the immune reaction. The AstraZeneca vaccine employs a chimpanzee adenovirus to carry DNA into recipients to spark an immune response against the coronavirus. Laboratory studies have suggested that the chimp virus or the DNA might cause the problem. Some researchers have suggested that bleeding from the injection, mixed with the vaccine, might put platelets in the cross-hairs of the immune system.

Dr. Greinacher called the theories plausible but unproven.

The article described specialized blood tests that can be used to diagnose the disorder, and distinguish it from other, more common clotting problems not related to the vaccine. The research team suggested treatment with a blood product called intravenous immune globulin, which is used to treat various immune disorders. Dr. Greinacher likened the treatment to putting out a fire.

Drugs called anti-coagulants, or blood thinners, can also be administered. But the researchers recommended against prescribing a commonly used one, heparin — because the vaccine-related condition is very similar to a severe reaction that occurs, rarely, in people given heparin.

The second report, from Norway, described five patients, one male and four female health care workers ages 32 to 54, who had clots and bleeding from seven to 10 days after receiving the AstraZeneca vaccine. Four had severe clots in the brain, and three died. Severe headaches were among their early symptoms. Like the German patients, all had high levels of antibodies that could activate platelets.

The team from Norway also recommended treatment with intravenous immune globulin. The researchers said the disorder was rare, but “a new phenomenon with devastating effects for otherwise healthy young adults,” and they suggested that it may be more common than previous studies of the AstraZeneca vaccine had indicated.

On Friday, European regulators also said they were reviewing reports of a few blood clot cases that occurred in people who had received the Johnson and Johnson vaccine. In the United States, federal agencies are investigating reports of a different type of unusual blood disorder involving a precipitous drop in platelets that emerged in a few dozen people who had received either the Pfizer-BioNTech or Moderna vaccines.

Benjamin Mueller and Melissa Eddy contributed.