The risk of developing blood clots after COVID-19 vaccination is very low, a study of 46-million adults in England has found.
The research showed that people vaccinated with Oxford-AstraZeneca or Pfizer-BioNTech had a lower risk of all arterial clots which can cause strokes and heart attacks, as well as and all venous clots.
A total of one to three extra people per million had an intracranial venous thrombosis after the Oxford-AstraZeneca vaccine.
However, there was no evidence that this was due to the vaccine.
Dr William Whiteley, project lead at the University of Edinburgh, said:
“Because of its very large size, this research study has provided precise results on the risks of rare blood clotting events and of low platelet levels following COVID-19 vaccination.
“We were able to show that these risks occur only in people under 70 years old with the Oxford-AstraZeneca vaccine and that the increase in risk is extremely small – no more than a few people per million vaccinated.”
The researchers analysed linked data from 46 million adults in England from the beginning of the COVID-19 vaccination programme on 8 December 2020 until 18 March 2021.
They looked for episodes of blood clots in the arteries and veins, as well as low levels of blood platelets that help with clotting in adults before and after vaccination and in unvaccinated adults.
The NHS health data included information from GPs, hospitals and death certificates, as well as Covid testing and vaccination data.
The study took into account other factors that could cause an association between vaccines and blood clots, such as age, sex, ethnicity and medical history.
The research was conducted by members of the CVD-COVID-UK consortium, a collaborative group of more than 260 members looking to understand the relationship between the coronavirus and cardiovascular diseases.
Dr Richard Francis, Head of Research at the Stroke Association, added:
“We are relieved to see that this research has shown that COVID-19 vaccines are unlikely to cause stroke except in fewer than 1-3 people in a million.
“Unfortunately, we already know that catching the virus without being vaccinated significantly increases your risk of a stroke.
“This research will help people to understand how their overall risk of stroke is affected by getting vaccinated. We look forward to the findings of research into why catching the virus increases your risk of stroke and how to prevent this.”
