“We have understood the mechanism of these thrombotic events, but the causes still need to be investigated”, explains the hematologist Sabine Eichinger, director of the anticoagulation clinic of the Medical University of Vienna and co-author of a recent study on post-vaccine thrombosis published in New England Journal of Medicine.
Are thrombotic events linked to an immune response?
“Yes. It is an antibody response. Of course the vaccine, being a vaccine, must induce an immune response. And therefore antibodies must be produced: but ideally they should only be produced against the coronavirus. Instead, fortunately only in some patients. , another immune reaction also occurs. The body produces antibodies not only against Sars-Cov-2, but also against a protein found in platelets, namely the platelet factor 4 (inside the platelets, it is found in the alpha granules). Here’s what happens: antibodies bind to platelet factor 4, in doing so they cause platelet activation, and activated platelets release additional platelet factor 4, so even more antibodies against platelet factor 4 are produced … it’s a kind of chain reaction. Activated platelets become aggressive, activate with each other, and also activate other parts of the blood clotting system. For example, plasma coagulation. And this causes a severe pre-thrombotic situation. Throughout this process the platelets are consumed, which is why a low number of platelets will be found in a blood test. This is because platelets are so active that they bind to anything, forming clots and wearing out in the process. “
So what happens to the platelets is certain. But what remains to be understood is why this happens (even if only in very rare cases) …
“There are several questions. We don’t know why in some people, after the vaccine, the immune system starts producing antibodies against platelet factor 4. What triggers this process? We don’t know. One hypothesis is that antibodies are produced against it. something that “resembles” platelet factor 4, and that there are therefore two different immune reactions. For example in the AstraZeneca or Johnson & Johnson vaccine there may be some additive that resembles – by some coincidence – platelet factor 4, and therefore the antibodies that occur form against that additive could then also attack – by reflex – the platelet factor 4.
Another hypothesis is that DNA fragments may be involved: it could be the DNA fragments in vaccines that stimulate the antibody response. Several research groups are currently investigating this. But these are only hypotheses that need to be investigated. And we also need to understand why this only happens to some people, and if these people have some characteristics in common “.
Perhaps there is something in common: for now it has been mainly women under 60 who have been affected by the rare thrombosis …
“It is too early to draw conclusions. We also need to consider these data from an epidemiological point of view. At least in the European Union, vaccination programs have started with vaccines other than AstraZeneca. And the program started with vaccinating older adults. . When vaccination with AstraZeneca started, nurses were vaccinated first. By definition they are young. And mostly women. So the data we have so far on thrombotic events may be influenced not so much by an intrinsic characteristic of thrombosis risk. how much from these choices that have been made upstream. This could be the reason why we see thrombosis cases occurring mainly among the under 60, and with a female prevalence. More interesting data should come from the United Kingdom, where many elderly people are been vaccinated with AstraZeneca: this will help us to better assess the thrombotic risk for the elderly entered by this type of vaccine “.
Could the fact that even with the Johnson & Johnson vaccine there have been rare thrombotic events indicate that the problem is the use of a virus as a vector?
“I do not have enough data on the Johnson & Johnson vaccine: even there it seems that the cases have mainly occurred in women. But I am not sure how the vaccination program was organized in the United States, or if priority was given to workers in the health sector. that even that is a vaccine that uses a virus as a vector is still an indication to consider, because so far mRNA vaccines such as those from Pfizer and Moderna have not triggered this type of rare thrombotic events “.
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