TORONTO – Scientists in Britain believe they have discovered the reason why some people experienced the very rare side effect of blood clots after receiving the AstraZeneca COVID-19 vaccine.

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Conclusion, published in the journal Science Advances On Wednesday, it is revealed that a protein found in the blood is drawn to a component of the vaccine, triggering a response from the body’s immune system that can result in clots.

The AstraZeneca vaccine, like other viral vector vaccines, uses a protected virus as a “host” to deliver genetic information about the COVID-19 spike protein to the vaccine recipient’s cells so that they can build up an immune response. and make antibodies.


The AstraZeneca COVID-19 vaccine uses a cold virus from a chimpanzee, also known as a chimpanzee adenovirus, as the delivery system. That’s because it can’t make humans sick – and, because they haven’t been exposed to it before, they won’t have antibodies to it.

Viral vectors modify the vaccine delivery virus so that it cannot spread, they only deliver instructions to the cells of the body.

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By analyzing the chimpanzee adenovirus used by AstraZeneca with cryo-electron microscopy, which allows for molecular-level detail viewing, the researchers found that the adenovirus binds to a blood protein known as “platelet factor 4”.

Cryo-electron microscopy involves flash-freezing the study subject and then bombarding it with electrons to produce microscope images of individual molecules. They are then used to recreate the structure of the subject in fine detail.

Cryo-electron microscopy image of the chimpanzee adenovirus that delivers the Oxford-AstraZeneca vaccine. (Science Advance Journal)

Platelet factor 4 is implicated in the genesis of another clotting phenomenon called heparin-induced thrombocytopenia (HIT), which causes changes in the blood that can lead to a clot forming in the blood vessels.

Researchers believe that the body begins attacking platelets attached to the adenovirus, after sensing it as part of a foreign virus. Then, when antibodies are released into the bloodstream to attack, they clump together with platelet factor 4 and can form a dangerous blood clot known as vaccine-induced thrombotic thrombocytopenia, or VITT.

Because that sequence of events in the body is so specific, it may explain why the development of blood clots after receiving a vaccine is such a rare event.

The UK findings echo a Canadian study conducted by researchers at McMaster University in Hamilton, Ont., and published this July. scientific journal nature,

In the Canadian study, researchers analyzed blood samples from five patients with VITT and compared them with blood samples from 10 patients with HIT and 10 samples from healthy individuals.

Looking at the amino acids of the interaction between the VITT antibody and the platelet factor 4 protein, the researchers found eight specific amino acids on the platelet protein that were identical to the binding site of HITT, which they believe led to similar patterns in the blood. can explain. Clots form after vaccination.

However the UK study AstraZeneca was able to present the highest resolution of the adenovirus used in the COVID-19 vaccine, allowing them to better understand its primary cell attachment protein and its structure.

VITT has been linked to at least one death in canada In May, an Ontario man received the AstraZeneca vaccine, and 73 of the nearly 50 million doses of AstraZeneca administered in the UK died.

The Public Health Agency of Canada’s National Advisory Committee stopped recommending AstraZeneca’s COVID-19 vaccine as a second-dose alternative in the spring of 2021 after some Canadian recipients reported blood clots.

Researchers in the UK hope that the findings of their study can be used to improve future vaccines and reduce the risk of blood clots.


With files from Brooklyn Neustetter, author of