One in 50,000 people under the age of 50 has developed a bleeding disorder after receiving an AstraZeneca vaccine. These are the data that emerged from the updates from England. While for the American College of Cardiology the cases of thrombosis found after the administration of Janssen’s Johnson & Johnson (always with a viral vector) would be about 20 cases out of nearly 18 million.
The mechanism linking some types of vaccine to the onset of extremely rare and atypical thrombosis is still highly uncertain. However, the main regulatory agencies (including Ema and Aifa) underline that this pathology, which doctors call Vaccine-induced immune thrombocytopenia (VITT) is one of the “very rare side effects” of vaccination. A few days ago, an article published in the scientific journal Nature lined up everything that researchers have discovered so far.
The thrombus
“Doctors and researchers have observed that following the administration of the two viral vector vaccines (Vaxzevria-AstraZeneca and Janssen by Johnson & Johnson), a thrombosis occurs in an infinitesimal percentage of people: that is, a clot is formed. of blood (thrombus) that hinders the circulation within the vessel itself causing more often “atypical” venous thrombosis, such as those of the cerebral veins or abdomen and more rarely of the arteries, such as cerebral infarction or stroke, heart attack of myocardium and pulmonary embolism, “he explains Corrado Lodigiani, Head of the Thrombosis and Hemorrhagic Diseases Center of Humanitas in Milan.
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It is difficult to understand who is most at risk
Unfortunately, to date, it is not possible to identify, before vaccination, the people who could potentially be more exposed to these adverse events. “In fact, there are no screening tests or useful pre-medications”. The expert, who urges everyone to get vaccinated, however, stresses that “thrombotic complications from SarsCoV-2 vaccine are very rare and absolutely inferior to the often fatal thrombotic complications that acute infection could cause”.
Although still under study, according to the article published in Nature, these could be the possible causes at the origin of the rare and atypical thrombosis:
The immune system responds abnormally
One of the plausible explanations is that this unusual syndrome is characterized by the association of thrombus and from a decrease in the platelet count derives from an abnormal immune response of the human organism towards the vaccine, with a peculiar involvement of the platelets.
This hypothesis arises from the similarity with an equally rare clinical condition sometimes seen in patients who are treated with heparin, an anticoagulant drug used for many decades to treat or prevent thrombosis.
This syndrome, called HIT (heparin-induced thrombocytopenia) is also characterized by a significant decrease in the number of platelets with simultaneous formation of clots and therefore thrombosis. “There is a probable similarity both from the point of view of the clinical manifestations and the biological mechanisms that are the basis”, explains Lodigiani. “In both cases, the events seem to be triggered by a reaction between a highly negatively charged molecule (in one case heparin and in the other – perhaps – adenovirus used as a vector for the Spike protein of the vaccine) and a molecule a positive charge which is on the surface of the platelets and which is called platelet factor IV, or PF4 (acronym of the English Platelet factor 4) “.
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Impurities remain in the vaccine
Thrombosis could be caused by some impurities left in the vaccines at the end of the production process – such as DNA fragments floating in the solution or protein residues in the broth used to grow the virus – which would interact, according to the article in Nature. with PF4 generating clots which are then targeted by antibodies. “This hypothesis is also plausible and this could explain the different incidence with the Johnson & Johonson vaccine which, despite being a viral vector, derives from a very different synthesis process and also requires only one administration and therefore less exposure to our immune system” , confirms the Head of the Thrombosis and Hemorrhagic Diseases Center of Humanitas.
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The administration
Possible factors include how the vaccines themselves may have been administered. In fact, the Nature article states that “Covid-19 vaccines are administered as injections into the muscle, but if the needle pierces a vein, the vaccine could enter the bloodstream directly. If this hypothesis were confirmed, many cases of VITT (vaccine-induced immune thrombocytopenia) could be avoided by asking vaccinators to first aspirate a small amount of fluid from the injection site with the syringe to rule out blood before actually pushing the plunger to administer the vaccine. “This is already standard practice in some countries and the Denmark has added it to its official guidelines for administering the Covid-19 vaccine. “In fact – confirms Lodigiani – it is a practice that in the past was suggested for vaccinations. In this case it would be supported by the fact that pre-clinical experiments have shown more thrombosis when the vaccine is administered intravenously. If the data were confirmed it would be a huge advantage “.
The cause, later denied, is instead linked to the Spike protein. “Initially it was thought that the antibodies capable of creating the immunocomplex with PF4 were those produced by the immune system against the spike protein itself but – concludes Lodigiani – experimentally it was then shown that these antibodies do not react with the protein and therefore it would not seem this is the responsible reaction of VITT (ed. vaccine-induced immune thrombocytopenia)“.
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