01 September 2021

The risk of blood clotting diseases caused by the COVID-19 vaccines is significantly outweighed by the risk of clotting diseases caused by infection with the COVID-19 virus, UK researchers said last week.

An Oxford University-led study, published in the BMJ, provides the first direct comparison of the risks of haematological and vascular illnesses in vaccinated and non-vaccinated people. Eight research institutions collaborated in the project.

Researchers compared 29 million people after their first dose with one of the UK’s two main vaccines – the adenoviral Oxford-AstraZeneca vaccine and the Pfizer-BioNTech mRNA vaccine – with 1.76 million people who had a positive test for infection with the SARS-CoV-2 virus.

They found an increased risk of thrombocytopenia (low platelet count), venous thromboembolism (VTE), and other rare arterial thrombotic events for short intervals after a first dose of the Oxford AstraZeneca vaccine and of arterial thromboembolism and ischaemic stroke for a short time after a first dose of the Pfizer-BioNTech vaccine. However, the risk of these adverse events were much higher following infection with SARS-CoV-2.  

Most significantly, cases of VTE occurred at a rate of 12,614 per ten million people infected with SARS-CoV-2– compared with 66 per ten million people who received the AstraZeneca adenoviral vaccine. For ischaemic stroke, they estimated there were 143 excess cases per ten million people who received the Pfizer vaccine, versus 1,699 excess cases for every ten million people infected with SARS-CoV-2.

Lead researcher Professor Julia Hippisley-Cox, a professor of general practice, said: “People should be aware of these increased risks after COVID-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2.”

She added: “This research is important as many other studies, while useful, have been limited by small numbers and potential biases. Electronic healthcare records, which contain detailed recording of vaccinations, infections, outcomes and confounders, have provided us with a rich source of data with which to perform a robust evaluation of these vaccines, and compare to risks associated with COVID-19 infection.”

Professor Aziz Sheikh, from the University of Edinburgh, said: “This enormous study, using data on over 29 million vaccinated people, has shown that there is a very small risk of clotting and other blood disorders following first dose COVID-19 vaccination.  Though serious, the risk of these same outcomes is much higher following SARS-CoV-2 infection.

“On balance, this analysis therefore clearly underscores the importance of getting vaccinated to reduce the risk of these clotting and bleeding outcomes in individuals, and because of the substantial public health benefit that COVID-19 vaccinations offer.”


Hippisley-Cox J, Patone M, Mei XW, Saatci D, Dixon S, Khunti K, Zaccardi F, Watkinson P, Shankar-Hari M, Doidge J, Harrison DA, Griffin SJ, Sheikh A, Coupland CAC. (2021) “Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study.” BMJ, doi: 10.1136/bmj.n1931.

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