AstraZeneca vaccine likely not linked to blood clots: review

The European Medicines Agency (EMA)’s Pharmacovigilance Risk Assessment Committee (PRAC), responsible for assessing all aspects of the risk management of medicines for human use, has concluded its preliminary review of a signal of blood clots in people vaccinated with COVID-19 Vaccine AstraZeneca, whose use was paused in more than 20 countries following reports of mostly young patients who suffered severe clotting disorders and rare types of strokes shortly after receiving the vaccine.

The committee stated that the vaccine is not linked with an increase in the overall risk of blood clots (thromboembolic events) in those who receive it; however, the vaccine may be associated with very rare cases of blood clots associated with thrombocytopenia, ie, low levels of blood platelets (elements in the blood that help it to clot) with or without bleeding, including rare cases of clots in the vessels draining blood from the brain (CVST). Overall, the committee concluded that the benefits of the vaccine in combating the still widespread threat of COVID-19 (which itself results in clotting problems) continue to outweigh the risk of side effects.

Around 20 million people in the UK and European Economic Area (EEA) had received the vaccine as of 16 March, with the EMA reviewing only seven cases of blood clots in multiple blood vessels (disseminated intravascular coagulation, DIC) and 18 cases of CVST. Overall the number of thromboembolic events reported after vaccination, both in studies before licensing and in reports after rollout of vaccination campaigns (469 reports, 191 of them from the EEA), was lower than that expected in the general population. This allows the PRAC to confirm that there is no increase in overall risk of blood clots, though in younger patients there remain some concerns.

The committee’s experts looked in extreme detail at records of DIC and CVST reported from member states, nine of which resulted in death. Most of these occurred in people under 55 and the majority were women. Because these events are rare, and COVID-19 itself often causes blood clotting disorders in patients, it was difficult to estimate a background rate for these events in people who have not had the vaccine. However, based on pre-COVID figures it was calculated that less than one reported case of DIC might have been expected by 16 March among people under 50 within 14 days of receiving the vaccine, whereas five cases had been reported. Similarly, on average 1.35 cases of CVST might have been expected among this age group whereas by the same cut-off date there had been 12. A similar imbalance was not visible in the older population given the vaccine.

The committee was of the opinion that the vaccine’s efficacy in preventing hospitalisation and death from COVID-19 outweighs the extremely small likelihood of developing DIC or CVST. However, in the light of its findings, patients should be aware of the remote possibility of such syndromes and, if symptoms suggestive of clotting problems occur, patients should seek immediate medical attention and inform healthcare professionals of their recent vaccination.

While at least a dozen countries have said they will restart vaccinations with the AstraZeneca vaccine, the PRAC says it will undertake additional review of the risks, including looking at the risks with other types of COVID-19 vaccines (although no signal has been identified from monitoring so far). Close safety monitoring of reports of blood clotting disorders will continue, and further studies are being instituted to provide more laboratory data as well as real-world evidence.

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