What are the risks of the AstraZeneca vaccine?

Currently, several EU countries are suspending use of the AstraZeneca vaccine, or recommending it not be used in various younger age groups, due to reports of blood clot risks. Australia has decided to delay rollout of vaccination for younger adults after restricting use of the AstraZeneca vaccine to people over 50. I was curious to see what the evidence said about the risks and the benefits, and here is a summary of my impressions. This is NOT a comprehensive or systematic review, just my take on the information I found

There are various population-based figures on the total incidence of deep vein thrombosis cases following vaccination with the AstraZeneca Covid-19 vaccine in the general population. The EU’s drug safety database had recorded 169 cases of blood clots in the brain, also known as cerebral venous sinus thrombosis, and 53 cases of those in the abdomen, or splanchnic vein thrombosis, in the EU and UK  — where some 34 million people had received the vaccine (1). That corresponds to an incidence of around 6 per million population.

Among more than 20 million people who have been vaccinated with the AstraZeneca vaccine in the UK so far, 79 cases of rare blood clots with low platelets have been reported, as well as 19 deaths according to the UK’s regulatoryauthority (2). This equates to around one case per 250,000 people vaccinated— or 4 per million—and one death in a million. EU officials have also reported that the number of “thromboembolic events” reported after vaccination was actually lower than expected in the general population (in 20 million people we would expect around 200 cases of deep vein thrombosis per week from other causes).

A group at Cambridge University has assessed the relative benefits and harms of the AstraZeneca vaccine (3). These are summarized in the three tables for population groups at low, medium and high risk of infection (UK infection rates). Presumably all Australians would be in the low risk group. The risk-benefit calculation looked at the intensive care admissions prevented versus potential blood clots. In all age and risk groups except low risk under 30s, the potential benefits far outstripped the potential harms. In people aged 20-29 and low risk—meaning that they have no conditions that make them more at risk of developing severe covid-19 illness—the harms slightly outweighed the benefits.

So there really is not any significant excess risk even for the young adult age group. A number of people have pointed out that use of oral contraceptives is associated with a substantially higher risk of blood clots, but that is still accepted as “very low risk”.

I took a look at the evidence on risk of blood clots associated with oral contraceptives, and systematic reviews have assessed the relative risk of deep vein thrombosis on combined oral contraception as 3 to 5-fold, resulting in an absolute risk of around 0.05% per year for a healthy adolescent (4-6). That corresponds to an incidence of 500 cases per million per year for oral contraceptive users compared to the risk of 4 per million following AstraZeneca vaccination. That risk is accepted by the medical profession and contraceptive users, and described in the literature as a “very low risk”.  In contrast, the AstraZeneca risk is substantially outweighed by the benefits in all except low risk your adults, where the risk and benefit is close to equal.

Its not clear to me that the evidence actually suggests there is an excess risk of mortality associated with the AstraZeneca vaccine, but lets assume there is indeed an extra risk of 1 death per million vaccinations.  The following table gives some comparative risks to put that in perspective. Most of these estimates derive from the UK population in 2010 or recent data for USA (7,8) and they are only intended to convey an indicative magnitude for various risks.

The risk of death from blood clots caused by the Astrazeneca virus are of the same order as the risk of dying by being struck by lightning and about half the risk of cycling 100 km.  That is indeed if there is a causal link and the clots are not just coincidental to the vaccination. In any case, the benefit seems to substantially exceed the harm for all groups except low risk adults under 30 years of age, where the risk of a blood clot (not death) is slightly greater than the risk of covid-19 infection resulting in hospital admission to ICU.


  • National Public Radio, 7 April 2021. EU Regulator: AstraZeneca Vaccine Effective; Blood Clots May Be A Rare Side Effect. ttps://www.npr.org/sections/coronavirus-live-updates/2021/04/07/984998679/eu-regulator-astrazeneca-vaccine-effective-blood-clots-may-be-a-rare-side-effect
  • Mahase E. AstraZeneca vaccine: Blood clots are “extremely rare” and benefits outweigh risks, regulators conclude BMJ 2021; 373 :n931 doi:10.1136/bmj.n931
  • https://wintoncentre.maths.cam.ac.uk/news/communicating-potential-benefits-and-harms-astra-zeneca-covid-19-vaccine/
  • Trenor CC 3rd, Chung RJ, Michelson AD, et al. Hormonal contraception and thrombotic risk: a multidisciplinary approach. Pediatrics. 2011;127(2):347-357. doi:10.1542/peds.2010-2221
  • Vandenbroucke JP, Rosendaal FR. End of the line for “third-generation-pill” controversy? Lancet. 1997 Apr 19;349(9059):1113-4. doi: 10.1016/S0140-6736(05)63015-2.
  • Lowe GD, Rumley A, Woodward M, Reid E. Oral contraceptives and venous thromboembolism. Lancet. 1997 May 31;349(9065):1623. doi: 10.1016/S0140-6736(05)61660-1.
  • Spiegelhalter, David; Blastland, Michael. The Norm Chronicles (p. 15). Profile. Kindle Edition.
  • Carly Hallman. Odds and Chances of Death by Activity and Behaviorhttps://www.titlemax.com/discovery-center/lifestyle/life-expectancy-by-activity-behavior/
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