Experts have found out why AstraZeneca vaccine may cause blood clotting disorders

Blood experts have discovered why the AstraZeneca vaccine causes blood clotting disorders in rare cases. There appears to be an extreme reaction of the immune system, which causes antibodies to be produced against its own platelets. The good news is that the condition can be detected and treated.

In recent weeks, more than 20 European countries have stopped vaccinations with the AstraZeneca vaccine after reports of a mysterious disease, a combination of thrombosis, and a reduced number of platelets. The safety committee of the EMA, the European Medicines Agency, eventually counted 25 cases: patients had to deal with clots in the brain or in the whole body after vaccination. Nine of them died.

EMA director Emer Cooke said on Thursday that there was no definitive evidence of a link between the vaccine and the rare condition. German, British and Italian doctors have now discovered the same antibodies in the blood of their patients. "It now seems clear to me that there is a connection with the vaccine," says professor Hugo ten Cate, thrombosis expert at Maastricht UMC. The risk of the side effect is extremely small, he emphasizes: it affects one in a million vaccinees.

The disease resembles a syndrome called HIT, in which the immune system is activated so strongly that the body produces antibodies against its own platelets. Platelets act as a kind of internal plaster, they form the first auxiliary forces in case of bleeding. The antibodies bind to the platelets, Ten Cate explains, and after contact with cells in the vessel wall, they subsequently contribute to the formation of blood clots. This creates an exceptional combination of too few platelets (causing bleeding) and thrombosis (which can clog the bloodstream).

More likely for women

On Friday evening, it became clear that the same clinical picture can also occur after vaccination (which is an artificially induced infection). British professor Marie Scully, her German colleague Andreas Greinacher and Italian thrombosis expert Marco Cattaneo discovered the antibodies in patients with the blood clotting disease who had recently been vaccinated with the AstraZeneca vaccine. "They informed all European colleagues in a short time," said Ten Cate.

It is not yet clear why the autoimmune disease mainly occurs in relatively young vaccinees, and especially in women. The vaccine may evoke a more powerful immune response in young people than in the elderly, says Ten Cate, and that also increases the chance that this reaction will go too far in them. Hormonal factors can play a role in women, he thinks. For example, it is known that the pill can sometimes also cause thrombosis. In the Netherlands, where 300 thousand AstraZeneca vaccinations have so far been applied, this exceptional condition has not yet been diagnosed. Last week, the Netherlands announced that it would resume usage of the AstraZeneca vaccine after it was temporarily halted.

American doctors have recently started reporting on patients who contract another rare blood clotting disease after inoculation with the Pfizer or Moderna vaccine. It concerns ITP, in which antibodies are also made against their own platelets. Professional publication The American Journal of Hematology came out on seventeen cases last month. It is still unknown whether the vaccine is the cause. But even then, that side effect would be extremely rare: In the United States, more than 20 million people have already been vaccinated with one of the two vaccines.