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Blood clot and stroke risks from vaccines ‘much lower’ than those from COVID-19

A study in the BMJ found there was much lower risk of the same adverse effects happening from vaccines than from COVID-19 infection
A study in the BMJ found there was much lower risk of the same adverse effects happening from vaccines than from COVID-19 infection Copyright Jae C. Hong/AP
Copyright Jae C. Hong/AP
By Luke Hurst
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A new study by the BMJ found there was an increased risk of rare blood clots associated with AstraZeneca and Pfizer’s vaccine, but that risk was much higher in patients who tested positive for coronavirus.


A new study has revealed the risk of blood clots from coronavirus vaccines is “much lower” than the risks from becoming infected with COVID-19.

The British Medical Journal (BMJ) study looked at data from more than 30 million people, finding that the risks of rare blood clotting events were “substantially higher and more prolonged after SARS-CoV-2 infection than after vaccination in the same population”.

More than 29 million people in the study were vaccinated with first doses, 19.6 million with AstraZeneca and 9.5 million with Pfizer, while 1.7 million people who had a positive COVID test were also looked at.

The scientists looked at the data to make estimates for how many adverse effects would occur in 10 million people, for each vaccine and then for COVID infection.

With the Astrazeneca vaccine there would be:

  • 107 cases of thrombocytopenia, which can cause internal bleeding
  • 66 cases of blood clots in the veins

With the Pfizer vaccine there would be:

  • 143 cases of stroke

With COVID-19 infection there would be:

  • 934 cases of thrombocytopenia
  • 12,614 cases of blood clots
  • 1,699 cases of stroke

The study found a minor increase in blood clots in those with the Pfizer jab, and a condition similar to a stroke in AstraZeneca, but the numbers were small and need further confirmation, the authors said.

“The small absolute increase in rates of adverse events reported by Hippisley-Cox and colleagues should be considered in the context of the proven benefits of Covid-19 vaccines and the risks of morbidity and mortality associated with SARS-CoV-2 infection,” said an accompanying editorial in the BMJ.

“For most, benefits far outweigh the risk of these rare adverse events; however, this balance will vary across specific subgroups (for example, based on age) and will also vary with population infection rates.”

It added that one key takeaway from the study was that it confirmed COVID-19 infection led to “substantially higher and more prolonged risks after infection than after vaccination”.

Professor of Clinical Epidemiology & GP at University of Oxford, Julia Hippisley-Cox, who co-authored the study told the BBC that while the research found there were increased risks from vaccines, “these are really reassuring results, in fact, to underscore the safety and benefits of the vaccine compared with the risk of getting an infection”.

A number of countries suspended the use of AstraZeneca’s vaccine earlier this year after possible links were found between the jab and rare blood clots.

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