How is the possible link between an anti-Covid vaccine and rare thrombosis studied?

Ema has commissioned some laboratories to investigate the possible causal relationship between viral vector vaccines against Covid-19 and the rare cases of thrombocytopenia: we need to have more data and understand the mechanism, also in view of future vaccines against coronavirus variants

(image: Pixabay) Not only Vaxzevria by AstraZeneca. Johnson & Johnson's (Janssen) anti-Covid vaccine, after reporting four cases of rare post-administration thrombocytopenia in the United States, also came under the magnifying glass of the European Medicines Agency (EMA), which asked companies to investigate further and provide new data and looks with interest at the research of two academic consortia in the Netherlands (at Erasmus University Medical Center in Rotterdam and at the University of Utrecht) to explore the possible triggering mechanism. Let's remember: at the moment the balance between risks and benefits is considered very much in favor of the vaccine, given that we are talking about very rare cases. There is much more risk with Covid-19 than with any side effects of vaccination.

Vaccine-induced immune thrombotic thrombocytopenia

As has been repeated several times by the authorities, the coagulation problems that in rare cases they appear to arise within 15 days of vaccination with the AstraZeneca Covid-19 vaccine are unusual: clots, thrombi form, in unusual parts of the body (brain and abdomen) and in (counterintuitive) conditions of thrombocytopenia, i.e. deficiency of platelets which are elements of the blood that promote clotting.

Is the causal link real?

The reported cases are so rare that it is difficult to understand if this is really a consequence of vaccination against Covid-19. A stronger suspicion has emerged from the latest expert analyzes, but Ema has requested to continue collecting data as the vaccination campaign continues. A warning: attention will now be focused on people who have just received the vaccination, but it is essential that there are no reports of unusual bleeding disorders among the unvaccinated population. Otherwise you risk inflating the relationship.

What mechanism?

Another crucial question that Ema asks to be answered is: why should a vaccine trigger such thrombocytopenia? One possible explanation stems from the fact that what these post-vaccination patients experience resembles heparin-induced thrombocytopenia (Hit), which can occur in people taking this anticoagulant. Hit appears to be caused by the body's immune response against the complex that is formed between heparin (which is a negatively charged molecule) and platelet factor Pf4 (which has a positive charge): unusual antibodies attack the complex and recall platelets , in a self-sustaining chain reaction that can cause blood vessels to clog.

According to some experts, something similar could happen with AstraZeneca's anti-Covid vaccine, which uses a viral vector ( i.e. a virus modified to carry the genetic material that codes for the coronavirus spike protein in the body): if the viral particles break before entering the cells, the genetic material they carry is released and being a negatively charged molecule could form a complex with the coagulation factor Pf4, triggering the immune response.

Where the causal link between vaccination with Vaxzevria and rare thrombosis was confirmed, understanding whether this is precisely the trigger mechanism of the adverse event would be very important because in theory it could also affect other viral vector vaccines, from J&J to Sputnik V, to Chinese vaccines.

Va underlined, however, that this is still a hypothesis and that possible causes could lie elsewhere, perhaps in an additive present in a product and not in others.

Are there predisposing factors?

Preliminary data collected from patients who underwent presumably vaccine-induced thrombocytopenia show the presence of anti-Pf4 antibodies, in fact. However, at the moment it cannot be said whether they pre-existed the anti-Covid vaccination. Indeed, some people are known to develop anti-Pf4 antibodies over their lifetime (perhaps as a result of other infections or perhaps as a result of some therapy, it is unclear), but these are controlled by an immunological mechanism known as peripheral tolerance . Could vaccination remove the brake of peripheral tolerance and trigger the action of anti-Pf4 antibodies?

The impression that rare adverse events of thrombocytopenia appear more frequent in women under 60 is also to be investigated further . For now, the data does not allow us to say anything about it, also considering the fact that in many countries the first phases of vaccination campaigns concerned the category of health workers, in which the female gender is predominant. And this may have altered the statistics.

Waiting for answers

There are many questions to be clarified, but the EMA expects to have some answers in the next two months, including tests on administration of half a dose of AstraZeneca vaccine, which long ago in a small number of volunteers had shown high efficacy in the face of a reduction in side effects.

Clarity is also essential for the development of future vaccines against Covid-19 , in view of new coronavirus variants.


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