How to explain the increased risk of thrombosis observed with the AstraZeneca vaccine? The Collective of the Side of Science, which brings together doctors, scientists, teachers and researchers involved “in information and prevention against covid-19 since the start of the pandemic“makes several hypotheses in an article published on March 25. He mentions in particular the track of an error in the administration of the vaccine.

An intramuscular vaccine

What error is this? This vaccine is administered intramuscularly in the deltoid muscle of the shoulder. Because a vaccine does not work by being disseminated in the blood. The immune reaction is local: it is the immune cells that travel to the injection site. But sometimes the needle may reach a vein rather than muscle tissue. It is this type of error that can pose a problem, according to the Collective.

To read also: What is a thrombosis?

Too many immune “threads” in the blood

The first hypothesis concerns an immune reaction. The AstraZeneca vaccine contains adenoviruses which have the S protein of SARS-CoV-2 on their surface. These “false viruses” trigger several immune reactions, including the production by some white blood cells of a kind of “net“called”NET“. Its goal is”“To catch” and “to imprison” circulating viruses“, explains the Collective.

However, according to several studies, NET may be associated with the occurrence of thrombosis in the event that the vaccine is accidentally injected into a vein. Those are “adenoviruses injected directly into the blood” which would result in the formation of NET in the vessels. Then the NETs would participate in ‘”thrombus formation“, blood masses coagulated in a vessel, then thrombosis, detail the authors of the article.

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Too much S protein on vessel cells

Another lead concerns the S protein, present on the adenoviruses of the vaccine. This protein could activate an immune cascade leading to thrombosis if it attaches to endothelial cells, those that line the inside of blood vessels.

How does it attach to it? As with all SARS-CoV-2 target cells: the S protein recognizes and attaches to ACE2 receptors in human cells.

However, if they are occupied by S proteins, less ACE2 is available on the surface of endothelial cells. And this phenomenon of withdrawal “could lead to an increased risk of thrombosis “, note the scientists, studies in support.

Check for the absence of “blood return”?

At the moment, these are only hypotheses that do not constitute scientifically proven evidence. But if the role of accidental intravascular injection in the occurrence of thrombosis was established, “a simple way“to overcome this risk”could be to check the absence of blood return during the vaccination, what the Ministry of Health does not currently recommend“say the authors of the article.

Checking that there is no “blood return” consists of pulling the syringe plunger once the needle has been placed. If blood goes back into the syringe, it means that the needle is not in the muscle but in a vessel and that it must be withdrawn, transplanted and the check repeated again. But this procedure is no longer used for intramuscular vaccines, because it can be painful and would not be of benefit.

The risk-benefit balance remains favorable

Whatever the cause of the increased risk of thrombosis observed with the AstraZeneca vaccine, its benefit-risk balance remains in any case largely favorable. “To put this data into perspective“, scientists point out that thrombosis occurs at a rare frequency of one in 200,000 people.

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In parallel, “the human cost of a sample of 1.5 million people infected (by covid, editor’s note) due to non-vaccination would be 1,500 deaths with a lethality of 1 per 1,000“.