The European Medicines Agency (EMA) and the United Kingdom (UK) Medicines and Healthcare products Regulatory Agency (MHRA) have completed preliminary reviews of the risk of blood clots in people vaccinated with the AstraZeneca COVID-19 vaccine. These reviews have found the benefits of the vaccine continue to outweigh the risks, and the vaccine is not associated with an increase in the overall risk of blood clots.
The number of blood clotting cases reported after vaccination, both in the large clinical trials undertaken prior to vaccine authorisation and during rollout of vaccination programs across the world, was lower than that expected in the general population. Around 20 million people in Europe, and several million more in other parts of the world, have received the vaccine.
The EMA safety committee noted there had been a very small number of cases of a rare type of blood clotting disease (disseminated intravascular coagulation) associated with low levels of blood platelets (blood cells that normally help blood to clot), with or without bleeding, as well as extremely rare cases of clots in the vessels draining blood from the brain (cerebral venous sinus thrombosis, CVST). It has not been proven that these cases were caused by the vaccine and a plausible mechanism for how they could have been caused by the vaccine has not been established. These conditions are also sometimes seen in people who have not received a COVID-19 vaccine or other vaccine. The EMA safety committee also noted that COVID-19 disease itself is often associated with serious clotting problems, which can themselves be fatal.
Additionally, the EMA found there was no evidence of a problem related to specific batches of the vaccine or to particular manufacturing sites.
The MHRA has also published a statement with its own separate review of reported cases and data from hospital admissions and GP records. They found that all the available evidence did not suggest common blood clots in veins were caused by the vaccine. The MHRA is undertaking a detailed review of the five cases of CVST with low blood platelets that occurred in the UK, and also notes that these events can occur naturally.
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