A McMaster University researcher has been instrumental in new research led by German scientists which has found that the AstraZeneca vaccine for COVID-19 may be rarely responsible for blood clots in people who have received the vaccine.

The researchers found the AZD1222 vaccine is associated with development of a blood clot disorder that resembles heparin-induced thrombocytopenia (HIT), or blood clots caused by the blood thinner, heparin, because of a decrease in the numbers of blood platelets, known as thrombocytopenia, caused by platelet-activating antibodies.

Ted Warkentin is a professor of medicine of the Michael G. DeGroote School of Medicine at McMaster and a hematologist. A co-author on the new paper, he was involved in developing the concept of the mechanism and helped to determine that these patients have unusual antibodies that strongly mimic those known to cause “spontaneous HIT syndrome.”

“This is an important discovery, because it establishes the explanation for this post-vaccination blood clotting disorder; it also means that a diagnosis can be easily established as shown by high levels of a particular type of HIT antibodies in the patient’s blood,” he said.

 “As well, a treatment tailored for this condition can be offered of blood thinners other than heparin, plus intravenous immunoglobulin to decrease the clotting tendency of these antibodies.”

He says clinicians should be aware that if symptoms or signs of possible thrombosis occur, for example affecting the brain or abdomen, beginning four to 20 days after vaccination, then a blood test should be performed to determine if there is a low platelet count (thrombocytopenia) which, if found, would indicate the need to perform further testing and treatment.

Warkentin is a hematologist at the Hamilton General Hospital, and also works at the McMaster Platelet Immunology Lab, where he and professor John Kelton originally discovered the subtype of HIT called “autoimmune HIT” 20 years ago. Together, they have made many important discoveries regarding HIT over the past 35 years.

The McMaster Platelet Immunology Lab, under the direction of Donald Arnold and Ishac Nazy, will be involved in the lab diagnosis of any cases of this vaccine-related adverse effect, should they occur in Canada.

The new paper was led by Andreas Greinscher, professor of the University of Greifswald and head of the Transfusion Medicine Department.

The research was published today in the New England Journal of Medicine


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