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Patient Evidence Summary
Doctor, does the AstraZeneca (ChAdOX1) or Pfizer (BNT162b2) vaccine against COVID-19 increase the risk of blood clots?
For most people who received a first dose of either the AstraZeneca or Pfizer vaccine, the risk of developing a blood clot was not higher than in people who remained unvaccinated.
For people less than 70 years of age, there was a very slightly increased risk of CVST with the AstraZeneca vaccine. This was not seen in people older than 70 or with the Pfizer vaccine at any age. This contributed to the decision to stop using the AstraZeneca vaccine in several countries.
Study highlights
This very large study showed that the the risk of developing blood clots is not higher in most people vaccinated with the first dose of the AstraZeneca or Pfizer vaccine compared to unvaccinated people. For people under the age of 70, about 1 to 3 more people out of one million, who received the AstraZeneca vaccine, developed a CVST compared to unvaccinated people. The risk of CVST was not higher in people with a history of blood clots or in people taking hormones.
Understanding the problem
Vaccination against the COVID-19 infection has been encouraged to prevent serious illness and death. In February 2021, some people who received the AstraZeneca vaccine developed blood clots in unusual locations including the veins around the brain (CVST) and even more rarely, in veins around the intestines or liver.
It has been challenging to identify how high the risk is for these complications, and who is at risk because of inconsistent reporting.
The National Health Service (NHS) organized the national vaccination program in England, which provided care for more than 46 million people. This means they had a very large database of medical records that they could check for complications due to vaccines and to determine who was more likely to develop blood clots.
Who? Adults 18 years of age or older who received a first dose of the AstraZeneca (ChAdOX1) or the Pfizer (BNT162b2) vaccine from December 8, 2020 to March 18, 2021.
What? Data gathered from electronic records belonging to the NHS in England.
After considering known risk factors for blood clots including age, sex, ethnicity, chronic illness, obesity, previous COVID-19 infection and medications, the researchers were able to calculate the risk associated with developing a blood clot following the first dose of vaccine.
Type of blood clot | Age | Risk of a blood clot 1 to 28 days after vaccination | Risk of a blood clot greater than 28 days after vaccination | Overall Conclusion |
|---|---|---|---|---|
Blood clot in a vein | <70 | No difference | No difference | The risk of a blood clot in a vein was not higher in vaccinated people compared to unvaccinated people. |
≥70 | Lower | Lower | ||
CVST (blood clot in veins around the brain) | <70 | Higher | No difference | The risk of CVST within 1 to 28 days of the AstraZeneca vaccine was slightly higher for people under age 70 compared to unvaccinated people (1 to 3 more people out of 1 million people). |
≥70 | No difference | No difference | ||
Blood clot in an artery | <70 | Lower | No difference | The risk of a blood clot in an artery was not higher in vaccinated people compared to unvaccinated people. |
≥70 | Lower | No difference |
Type of blood clot | Age | Risk of a blood clot 1 to 28 days after vaccination | Risk of a blood clot greater than 28 days after vaccination | Overall Conclusion |
|---|---|---|---|---|
Blood clot in a vein | <70 | Lower | Lower | The risk of a blood clot in a vein was not higher in vaccinated people compared to unvaccinated people. |
≥70 | Lower | Lower | ||
CVST (blood clot in veins around the brain) | <70 | No difference | No difference | The risk of CVST was not higher in vaccinated people compared to unvaccinated people. |
≥70 | No difference | No difference | ||
Blood clot in an artery | <70 | Lower | Lower | The risk of a blood clot in an artery was not higher in vaccinated people compared to unvaccinated people. |
≥70 | Lower | Lower |
This Evidence Summary is based on the following article:
Whiteley WN, Ip S, Cooper JA, et al. Association of COVID-19 vaccines ChAdOx1 and BNT162b2 with major venous, arterial, or thrombocytopenic events: A population-based cohort study of 46 million adults in England. PLoS Med. 2022 Feb 22;19(2):e1003926. doi: 10.1371/journal.pmed.1003926. eCollection 2022 Feb. PubMed
Natasha Sheliza Rupani MD, FRCPC
Dr. Rupani is currently pursuing the The Alexandra Yeo Fellowship in Thrombosis and Hemostasis at the University of Toronto. She is concurrently supported by a CanVECTOR Fellowship Award and is completing a Master of Science in Quality Improvement and Patient Safety at the University of Toronto. Her research interests include evaluating the management of cancer-associated thrombosis in special populations and using a quality lens to to prevent and reduce venous thromboembolism in the cancer population.
Lori-Ann Linkins, MD, MSc (Clin Epi), FRCPC
Dr. Linkins is an Associate Professor of Medicine (thrombosis) at McMaster University in Hamilton, Canada. She holds a Masters Degree in Health Research Methodology and is a Deputy Editor with the Health Information Research Unit, McMaster. She is Co-Editor of the ACP Journal Club and Co-lead on the CanVECTOR Knowledge Translation Platform.
Published: Thursday, May 5, 2022
Please note that the information contained herein is not to be interpreted as an alternative to medical advice from a professional healthcare provider. If you have any questions about any medical matter, you should consult your professional healthcare providers, and should never delay seeking medical advice, disregard medical advice or discontinue medication based on information provided here.
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, McMaster University