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People Are Eight Times More Likely To Get Blood Clots From COVID-19 Than AstraZeneca Vaccine, Study Finds


Tom Hale

Senior Journalist

clockApr 15 2021, 15:36 UTC

Unusual blood clots have been reported in about 5 in a million people after first dose of the AZ-Oxford COVID-19 vaccine. Image credit: Prostock-studio/

There’s been a lot of news at the moment about COVID-19 vaccines and the low risk of blood clots. A new preliminary study has put things in perspective: the risk of developing a blood clot from having COVID-19 is eight times higher than the risk of developing one after receiving the Oxford-AstraZeneca vaccine. 

Scientists at the University of Oxford compared the risk of rare blood clotting in the brain, known as cerebral venous thrombosis (CVT), following COVID-19 to the risk after vaccination (including mRNA vaccines, such as Pfizer and Modern, and the AstraZeneca-Oxford vaccine). 


The key finding was that the risk of CVT from COVID-19 is about eight times greater compared to the AstraZeneca-Oxford vaccine, while the risk of a CVT from COVID-19 is about 10 times greater compared to the mRNA vaccines.

Here's how the instances of blood clots broke down:

  • In this study of over 500,000 COVID-19 patients, CVT occurred in 39 in 1 million patients.
  • In over 480,000 people receiving a COVID-19 mRNA vaccine (Pfizer or Moderna), CVT occurred in 4 in 1 million.
  • CVT was reported in about 5 in 1 million people after first dose of the AZ-Oxford COVID-19 vaccine.

The study has not yet been peer-reviewed, but given the timely nature of the debate, it’s been published as a preprint paper, available to read here.

There’s been a huge amount of controversy in recent weeks regarding the risk of blood clots after having certain COVID-19 vaccines. Primarily the focus has been on the AstraZeneca vaccine, but this week also saw the US pausing the use of the Johnson & Johnson Janssen vaccine after six cases of rare blood clots were reported. While much of the debate surrounding whether the risk outweighs the benefits is ongoing, the UK and European health authorities concluded that the benefits of receiving the Oxford-AstraZeneca COVID-19 vaccine very favorably outweigh the potential risks for the vast majority of people.


To highlight the relatively low risk, many have been making comparisons with the risk of blood clots from other drugs, most notably the contraceptive pill. Blood clots arise from the pill in 4 to 16 out of 10,000 people each year.  

However, it can be slightly misleading to make such direct comparisons. This is because the risk is multifaceted and highly complex depending on the person and mechanism underlying the formation of these clots, making some comparisons reductive. This is also a criticism which some experts in the field have already launched towards this new research.

“The major issue here is that the comparison showing the higher risk after Covid-19 does not exclude the possibility the pathogenesis is the same and therefore some common denominator should be searched,” Professor Paolo Madeddu, Professor of Experimental Cardiovascular Medicine at the University of Bristol in the UK, commented.

“For instance, if the mechanism is the same, one can speculate that the high occurrence in COVID-19 vs vaccination is because the whole virus is more thrombogenic than the spike protein alone,” Professor Madeddu added. “These studies are important but seem to be focused on demonstrating the minor risk of vaccination instead of making efforts to explain the cause of complications, taking advantage of the similarities of the events in the two populations. “


In sum, the preliminary findings should be interpreted with caution, as the researchers readily admit data is still accruing, but it does highlight how the risk of having a COVID-19 vaccine to protect against the disease has a lower risk of developing a blood clot than COVID-19 itself.

"Overall the main finding is that these CVT events are very rare – a few in every million people involved – in Covid-19 patients and in people who had one of the vaccines – but they were very much rarer in the people who had a vaccine than in people who had Covid-19," explained Professor Kevin McConway, Emeritus Professor of Applied Statistics at the Open University, who was not directly involved in the research.

For more information about COVID-19, check out the IFLScience COVID-19 hub where you can follow the current state of the pandemic, the progress of vaccine development, and further insights into the disease.

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