McMaster University researchers know exactly why some vaccines can cause blood clots


Published July 7, 2021 at 4:18 pm


A team of researchers at McMaster University in Hamilton found the cause of rare but sometimes fatal blood clots caused by certain vaccines.

The blood clotting reaction is known as vaccine-induced immune thrombotic thrombocytopenia (VITT). According to researchers, it’s much more severe than a typical blood clot because veins that drain blood from the brain are obstructed and can potentially cause fatal bleeding.

It’s believed that the findings will put scientists on the path of finding a way to better diagnose and treat VITT, possibly prevent it, and potentially make vaccines safer.

“Our work also answers important questions about the connection between antibodies and clotting,” said Ishac Nazy, principal investigator and corresponding author of the study.

The AstraZeneca and Johnson and Johnson CSARS-CoV-2 (COVID-19) vaccines use adenoviral vectors, which are designed to shuttle a gene from COVID-19 into our bodies where our cells will read it and make coronavirus spike proteins. Adenoviral vectors can, however, cause the antibodies to stick to components from blood platelets — triggering the formation of a clot.

“The antibodies stick to the platelet protein called platelet factor 4 (PF4) in a very unique and specific orientation, which allows them to align with other antibodies and platelets in the precise formation that leads to a self-perpetuating vicious cycle of clotting events,” added Nazy, who is also the scientific director of the McMaster Platelet Immunology Laboratory and an associate professor of medicine for the Michael G. DeGroote School of Medicine at McMaster.

“These disease-causing aggregates quickly activate platelets, creating a highly intense clotting environment in patients.” 

The dangerous reaction to the adenovirus vector vaccines has been found to occur in one in 60,000 people receiving the vaccine in Canada.

“The intention of our study was to better understand how the severe clots which characterize VITT develop,” said Donald Arnold, study co-investigator and co-medical director of the McMaster Platelet Immunology Laboratory.

“A basic principle of medical care is to understand how the disorder happens and, in doing so, develop better treatments.”

Researchers say the next step is to develop a rapid diagnostic and accurate test to diagnose VITT. From there, it’s believed researchers can find a way to prevent the clot from happening altogether.

The research team included Angela Huynh, a research scientist in the McMaster Platelet Immunology Laboratory; Mercy Daka, a graduate student of McMaster’s Department of Biochemistry and Biomedical Sciences; and John Kelton, co-medical director of the McMaster Platelet Immunology Laboratory.

The research can be found here:

Cover Photo: Authors of the paper, from left to right, Ishac Nazy, Angela Huynh, John Kelton, Donald Arnold and Mercy Daka (Photo by James Smith).

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