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Incidence of very rare thromboembolic events was far lower than in people diagnosed with COVID-19 infection

In a large real-world study, data published as a pre-print on The Lancet server from over one million individuals assessed the incidence rates of blood clotting disorders of thromboembolism and thrombocytopenia, including the very rare thrombosis with thrombocytopenia (TTS) following vaccination with an mRNA vaccine or Vaxzevria, and compared them with expected rates in a general population and in people with COVID-19.

Safety profiles of Vaxzevria, and the mRNA-based vaccine were similar and overall favourable. Very rare clotting disorders (TTS) were observed with both vaccines but these were in line with what would be expected in the general population and lower than in those diagnosed with COVID-19.1 Follow-up time was not sufficient to report rates after two doses of Vaxzevria, though other studies have shown rates of rare blood clotting events to be lower after a second dose.2

Regardless of the vaccine used, the increase in rates of thrombosis among persons infected with COVID-19 was far higher than among those vaccinated. Rates of venous thromboembolism were eight times higher after a diagnosis of COVID-19 infection compared with the expected rate.

Sir Mene Pangalos, Executive Vice President, BioPharmaceuticals R&D, said: “This real-world study offers further evidence of the favourable benefit:risk profile of Vaxzeria and demonstrates the critical role all COVID-19 vaccines are playing in combatting the pandemic.”

The analysis included 945,941 mRNA participants (778,534 with two doses), 426,272 Vaxzevria participants, conducted between 27 December 2020 and 19 May 2021. It also included 222,710 COVID-19 participants identified between 1 September 2020 and 1 March 2021, and 4,570,149 background participants as of 1 January 2017 from a public health database held in Catalonia, Spain.

The results are in line with recent reports in the Medicines and Healthcare products Regulatory Agency (MHRA) Yellow Card Report, the UK system for collecting and monitoring information on safety concerns, which also show low rates of TTS after a second dose.2

No specific risk factors or definitive cause for TTS following COVID-19 vaccination have been identified and AstraZeneca continues to perform and support ongoing investigations of potential mechanisms. Furthermore, these very rare events can be managed when symptoms are identified and treated appropriately.3

Vaxzevria, formerly AZD1222
Vaxzevria was co-invented by the University of Oxford and its spin-out company, Vaccitech. It uses a replication-deficient chimpanzee viral vector based on a weakened version of a common cold virus (adenovirus) that causes infections in chimpanzees and contains the genetic material of the SARS-CoV-2 virus spike protein. After vaccination, the surface spike protein is produced, priming the immune system to attack the SARS-CoV-2 virus if it later infects the body.

The vaccine has been granted a conditional marketing authorisation or emergency use in more than 80 countries across six continents. More than 800 million doses of COVID-19 Vaccine AstraZeneca have been supplied to more than 170 countries worldwide, including more than 100 countries through the COVAX Facility.

AstraZeneca (LSE/STO/Nasdaq: AZN) is a global, science-led biopharmaceutical company that focuses on the discovery, development, and commercialisation of prescription medicines in Oncology, Rare Diseases and BioPharmaceuticals, including Cardiovascular, Renal & Metabolism, and Respiratory & Immunology. Based in Cambridge, UK, AstraZeneca operates in over 100 countries and its innovative medicines are used by millions of patients worldwide. Please visit and follow the Company on Twitter @AstraZeneca.

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Last Updated: 28-Jul-2021