10 May 2021

A population study in two Scandinavian countries of the Oxford-AstraZeneca vaccine has found 11 excess venous thromboembolic events for every 100,000 vaccinations.

The analysis, published last week in the British Medical Journal (BMJ), confirms the rarity and nature of the events. However, experts say it is still hard to compare the vaccine risks with the risks of unfettered spread of COVID-19.

According to one expert, without vaccination, the population would face significantly increased risks of thrombotic events from viral infection.

The study is based upon over 280,000 people in Norway and Denmark who received the first dose of the Oxford-AstraZeneca vaccine. Both countries have suspended the use of the vaccine.

The researchers found 59 venous thrombotic events compared with 30 that would have been expected in a similar group by age and sex. There was no additional risk of arterial thrombosis.

Risk of cerebral venous thrombosis was particularly raised; there were 2.5 incidents of cerebral venous thrombosis for every 100,000 vaccinations, which represents a more than 20-fold relative risk increase. Other events included thrombocytopenia and incidents of bleeding and clotting disorders – which, the researchers say, might have simply been highlighted by increased surveillance.

The researchers, led by Professor Anton Pottegård, a public health specialist at the University of Southern Denmark, write: “The absolute risks of venous thromboembolic events described in this study are small, and the findings should be interpreted in the context of the benefits of COVID-19 vaccination at both the societal and the individual level.”

Writing an opinion piece in the BMJ, Professor Paul Hunter, of the University of East Anglia, UK, says the findings support the conclusions of UK and EU regulators that the benefits of the vaccine outweigh its risks for most age groups.

He writes: “Those countries that delayed their own vaccination programmes at a time of high transmission rates by declining to use available Oxford-AstraZeneca vaccines should know that their decision will have contributed to an increase in the number of avoidable deaths from COVID-19.”

And the BMJ editors Rafael Perera and John Fletcher write in an editorial: “The choice we nearly all face is between eventual SARS-CoV-2 infection or vaccination. The Oxford-AstraZeneca vaccine is clearly a good choice, despite the likely risks [reported in this study]. Quantifying the comparative risk associated with other vaccines is now a research (and public health) priority.”


Pottegård A, Lund LC, Karlstad Ø, Dahl J, Andersen M, Hallas J, Lidegaard Ø, Tapia G, Gulseth HL, Ruiz PL, Watle SV, Mikkelsen AP, Pedersen L, Sørensen HT, Thomsen RW, Hviid A. (2021) “Arterial events, venous thromboembolism, thrombocytopenia, and bleeding after vaccination with Oxford-AstraZeneca ChAdOx1-S in Denmark and Norway: population based cohort study.” BMJ, doi: 10.1136/bmj.n1114


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