AstraZeneca shot linked to 30 pc higher risk of rare blood clots compared to Pfizer: Global study

Research says although rare, observed risks after adenovirus-based vaccines should be considered when planning further immunisation campaigns and future vaccine development

Photo for representation. Reuters

Tribune News Service

New Delhi, October 27

A comparative multinational study on rarest blood clotting condition after the receipt of Covid 19 vaccines has revealed a 30 per cent increased risk of thrombocytopenia (a potentially risky condition that produces rare blood clots with low platelet levels) after a first dose of the Oxford-AstraZeneca vaccine as compared with the Pfizer-BioNTech shot.

The research titled “Comparative risk of thrombosis with thrombocytopenia syndrome or thromboembolic events associated with different covid-19 vaccines: international network cohort study from five European countries and the US” published in the British Medical Journal on Thursday concluded that although rare, the observed risks after adenovirus-based vaccines (AstraZeneca and Johnson and Johnson’s Janssen) should be considered when planning further immunisation campaigns and future vaccine development.

The study aimed to quantify the comparative risk of thrombosis (when blood clots block veins) with thrombocytopenia syndrome or thromboembolic events associated with use of adenovirus based covid-19 vaccines (AstraZeneca and Janssen) versus mRNA based covid-19 vaccines (Pfizer and Moderna).

Overall, 13,32,719 of 38,29,822 first dose AstraZeneca shot recipients were matched to 21,24,339 of 21,49,679 Pfizer shot recipients from Germany and the UK.

Additionally, 7,62,517 of 7,72,678 people receiving Janssen vaccine were matched to 28,51,976 of 76,06,693 receiving Pfizer in Germany, Spain, and the US.

“All 6,28,164 AstraZeneca recipients from the US were matched to 22,30,157 of 39,23,371 Moderna shot recipients.

“A total of 862 thrombocytopenia events were observed in the matched first dose AstraZeneca recipients from Germany and the UK, and 520 events after a first dose of Pfizer shot. Comparing AstraZeneca with a first dose of Pfizer revealed an increased risk of thrombocytopenia. Additionally, higher incidence rate of venous thrombosis with thrombocytopenia syndrome was seen with Janssen adenovirus vaccine compared with Pfizer, which is a messenger RNA shot.

Thrombocytopenia is a condition in which people have a low blood platelet count.

These are extremely rare events, and are more likely to occur with adenovirus vector than mRNA vaccine, researchers said.

Adults from France, Germany, the Netherlands, Spain, the UK, and the US who received at least one dose of a covid-19 vaccine (ChAdOx1-S (Oxford-AstraZeneca), BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), or Ad26.COV2.S (Janssen/Johnson & Johnson)), from December 2020 to mid-2021 were covered by the study.

India’s Serum Institute of India mass produced the AstraZeneca shots which were marketed as Covishield in India.

Fewer concerns over safety of mRNA vaccines

By May 2021, the above mentioned four vaccines had been granted conditional marketing authorisation by the European Medicines Agency. After millions of vaccine doses were given in large scale immunisation campaigns, rare cases of thrombosis with thrombocytopenia syndrome were reported, often after the first dose of adenovirus vaccines. Although fewer concerns have been raised about the safety of mRNA vaccines, instances of immune thrombocytopenia have also been observed in recipients of Pfizer shots.

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