After reports of blood clots experienced by some people after receiving the AstraZeneca Covid-19 vaccine triggered safety concerns, new research published in a leading medical journal suggests that a rare antibody reaction was found in a few people who got the vaccine.

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The reports which looked into the issue of blood clots linked to AstraZeneca jabs were published in The New England Journal of Medicine. The results showed that among the studied population, those who developed the clots after vaccination had produced antibodies that activated their platelets, a blood component involved in clotting, The New York Times reported last week. However, it is not yet clear why the rare reaction occurred.

In one of the reports, the researchers assessed the clinical and laboratory features of 11 patients in Germany and Austria in whom blood clots had developed after vaccination with the AstraZeneca vaccine. The researchers used a standard enzyme-linked immunosorbent assay to detect platelet factor 4 (PF4)-heparin antibodies and a modified (PF4-enhanced) platelet-activation test to detect platelet-activating antibodies under various reaction conditions. The researchers concluded that vaccination with "ChAdOx1 nCov-19" can result in the rare development of immune thrombotic thrombocytopenia mediated by platelet-activating antibodies against PF4, which clinically mimics autoimmune heparin-induced thrombocytopenia.

The other study by a team from Norway presented five cases of severe venous thromboembolism in unusual sites and concomitant thrombocytopenia that occurred 7 to 10 days after vaccination for COVID-19. Meanwhile, a trial of the Oxford-AstraZeneca COVID-19 vaccine on children in the UK was earlier paused even as the country's medicines regulator probes a possible link of the vaccine with rare blood clots in adults, the media reported. The European Medicines Agency had said that the vaccine's labelling should be revised to include listing the clotting disorder as a "very rare" side effect of the vaccine.

In a statement on its website, AstraZeneca said it was "actively collaborating with the regulators to implement these changes to the product information and is already working to understand the individual cases, epidemiology and possible mechanisms that could explain these extremely rare events".