Astra Zeneca vaccine: antibodies against platelets explain thrombosis cases

After repeated alarms for the rare cases of thrombosis that have occurred in patients after receiving the AstraZeneca vaccine, there is now a possible scientific explanation for these events.

The study, supported by the German Research Foundation, and whose first author is Dr. Greinacher of the University of Greifswald, was recently published in the New England Journal of Medicine.

Cerebral venous thrombotic events

The AstraZeneca COVID-19 vaccine is a vector vaccine with adenovirus that received a marketing authorization in the European Union on January 29, 2021. In the following months, very rare thromboembolic events have been reported in several countries following its administration, enough to lead in some cases to the suspension of some lots or the entire supply.

A first analysis conducted by the EMA, the European Medicines Agency, of 8 March 2021 had not actually identified any increase in the risk of thromboembolic events following the administration of the vaccine. However, the reported cases have slowly grown, while remaining confined to very few patients.

A few days ago, the Agency’s safety committee concluded that thrombosis cases detected, in the presence of a low platelet count, should be listed as very rare side effects of the AstraZeneca vaccine.

These events usually occurred within two weeks of vaccination, most of them occurring in women under the age of 60. The blood clots occurred in the veins of the brain and abdomen, but also in arteries and were associated with low platelet levels and sometimes bleeding.

As a possible explanation, the Agency has proposed an immune response, which leads to a condition similar to that sometimes seen in patients treated with heparin-induced thrombocytopenia.

A new study now seems to bring more precise information on the pathological phenomena induced by the vaccine, confirming the EMA hypotheses.

A reduction in platelets

The trial involved 11 patients who developed thrombosis or thrombocytopenia after vaccination with the AstraZeneca vaccine.

Of the 11 patients, 9 were women, with a mean age of 36 years. Thrombotic events occurred over a range of 5 to 16 days after vaccination. Patients experienced one or more thrombotic events, while in one case fatal intracranial hemorrhage occurred. The venous thrombosis had localized to the cerebral and splanchnic levels. In some cases, pulmonary embolism had occurred. Death occurred in six cases. Some patients have had disseminated intravascular coagulation.

The authors therefore conclude that in rare cases the vaccine can cause thrombotic thrombocytopenia due to drug-induced antibodies.

Antibodies to platelets

In summary, this new study seems to indicate that the AstraZeneca vaccine not only induces the production of antibodies that target the SARS-CoV-2 virus, but also different antibodies, which target PF4.

PF4 is a protein found in platelets and is normally linked to the heparin produced by our body. Its task is to maintain the right balance to have blood fluidity in normal conditions and effective haemostasis in the event of injuries.

When the antibodies generated by the vaccine mistakenly hit PF4, the number of circulating platelets is reduced, but these are also activated, to promote the formation of thrombus.

However, reported cases of these events remain very rare compared to the millions of vaccine doses administered. On the other hand, the fact that there has been so much attention to these events testifies how efficient and rigorous pharmacovigilance process is. A process that puts all drugs on the market under continuous control and that guarantees us to receive only safe treatments. Agencies such as EMA and FDA work for this and combine data and expertise to ensure the safety of citizens.

Moreover, it is inevitable that the great resonance that these cases have had in the media lead to a reduction in people’s trust in vaccines, not only in AstraZeneca, unfortunately. A phenomenon that must absolutely be countered to avoid much worse damage to public health.

If the AstraZeneca vaccine against COVID-19 is completely blocked, perhaps we could have a few dozen less cases of venous thrombosis, but certainly thousands more deaths from the virus.

This post is also available in: Chinese (Simplified)

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