Concerned about blood clots and the AstraZeneca COVID-19 Vaccine? Make sure you are well informed of the facts before you make any decisions!

The Oxford-AstraZeneca vaccine was recently suspended in several countries in those under the age of 60 due to concerns of very rare types of blood clotting, called cerebral sinus venous thrombosis (CVST). This very rare clotting condition is associated with low blood platelets (thrombocytopenia). Last week, The European Medicines Agency’s (EMA) safety committee concluded that unusual blood clots with low blood platelets should be listed as very rare side effects of Vaxzevria (formerly COVID-19 Vaccine AstraZeneca) [1].

In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) examined 79 cases of this unusual clotting up to the 31st of March, 19 of these were fatal [2]. This was out of 20.2 million vaccine doses, making the risk of these clots approximately 4 per million doses (or 1 per 250,000 doses) [2]. We have not seen cases of CVST until recently in the UK because CVST is more common in younger adults and we have only recently started vaccinating younger adults in abundance [3].

Why wasn’t this picked up in clinical trials? Well, rare vaccine side effects may occur in less than 1 case per tens of thousands of vaccine doses [4]. Our clinical trials are not designed to be large enough to detect extremely rare side effects, their purpose is to test for efficacy and common side effects [4]. Trials also typically recruit healthy individuals rather than those with underlying health conditions. Now that tens of millions of people are receiving the vaccine, rare side effects are bound to reveal themselves [4].

About 100,000 people usually develop blood clots each month in the EU, and 3,000 cases a month are thought to occur in the UK [5]. Let us put this into context a bit more:

  • 1 in 250,000 people vaccinated with the AstraZeneca vaccine will develop blood clots with low platelets [5].
  • 1 in 2000 women each year will develop a blood clot from taking the combined oral contraceptive pill [6].
  • 1 in 1000 people a year will develop a blood clot from air travel [7].

It has been concluded that no age restrictions were necessary for the vaccine, the UK’s vaccine advisory committee has decided that adults under 30 who are healthy and not at risk of developing severe COVID-19 should be offered an alternative vaccine where possible [5]. Wei Shen Lim, chair of the UK’s Joint Committee on Vaccination and Immunisation, said, “We are not advising a stop to any vaccination for any individual in any age group. We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution rather than because we have any serious safety concerns” [5].

It should be noted that COVID-19 itself also presents a significant risk of developing a blood clot [5]. One study found that, in people with COVID-19, the overall prevalence of pulmonary embolism was 7.8% and deep vein thrombosis 11.2%. Of those who ended up in intensive care, 23% developed venous thromboembolism [5]. COVID-19 also led to strokes in around 1.6% of people, and an estimated 30% of people with COVID-19 will get thrombocytopenia, a lowering of the platelet count. Munir Pirmohamed, chair of the UK’s Commission on Human Medicines, said, “That puts into context the fact that the risk of clots and lower platelets is much higher with COVID-19 than these extremely rare events which are occurring with the vaccine” [5].

It is crucial to ensure you have sufficient knowledge on these matters before making decisions about whether to get vaccinated, and we hope you feel more informed about this important topic after reading this article.

Mia Georgiou


[1] European Medicines Agency. 2021. AstraZeneca’s COVID-19 vaccine: EMA finds possible link to very rare cases of unusual blood clots with low platelets – European Medicines Agency. [online] Available at: <> [Accessed 12 April 2021].

[2] GOV.UK. 2021. MHRA issues new advice, concluding a possible link between COVID-19 Vaccine AstraZeneca and extremely rare, unlikely to occur blood clots. [online] Available at: <> [Accessed 12 April 2021].

[3] 2021. [online] Available at: <> [Accessed 12 April 2021].

[4] Remmel, A., 2021. Why is it so hard to investigate the rare side effects of COVID vaccines?. Nature,.

[5] Mahase, E., 2021. AstraZeneca vaccine: Blood clots are “extremely rare” and benefits outweigh risks, regulators conclude. BMJ, p.n931.

[6] McCartney M. Medicine: before COVID-19, and after. Lancet2020;395:1248-9. doi:10.1016/S0140-6736(20)30756-X pmid:32243779

[7] National Institute for Health and Care Excellence. DVT prevention for travellers: summary. Revised Aug 2018.,is%20prolonged%20seating%20and%20immobility.

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