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Patient Evidence Summary

Doctor, I have a history of blood clots. Is it safe for me to receive a vaccine for COVID?

There is no evidence that people with a history of blood clots or a family history of blood clotting disorders are at higher risk for developing the rare blood clots that have been reported after some vaccines for COVID. For more information on COVID vaccines and blood clots, please see Thrombosis Canada and COVID 19

Study highlights

This study describes the symptoms and diagnostic test results in 11 people who presented with a rare blood clotting problem after COVID vaccination in Germany and Austria. By identifying the cause of this unusual clotting syndrome, this study was a very important step forward in helping clinicians diagnose and treat this rare condition.

Understanding the problem

About 1 in 40,000 to 1 in 250,000 in people who have received the AstraZeneca* vaccine have been diagnosed with an unusual blood clotting syndrome between 4 to 28 days after vaccination. Unlike typical blood clots that are triggered by the clotting system, these blood clots are triggered by the immune system. That means the usual risk factors for increasing the risk of clotting do not increase the risk of these particular clots, based on what we know to date. This study was one of the first to report this rare blood clotting problem called vaccine-induced immune thrombotic thrombocytopenia (VITT)** and to describe what appears to be causing it.

*There have been similar reports for the Johnson and Johnson vaccine

**previously known as VIPIT


11 people in Germany and Austria were found to have one or more blood clots 5 to 16 days after receiving an AstraZeneca vaccine for COVID. Nine of the 11 people were women (age range 22 to 49 years). Only one of them had a history of a medical problem that could be related to clotting and only one of them was taking a birth control pill. (Taking the birth control pill is known to increase the risk of forming typical blood clots.)


These people shared specific features that gave doctors clues that something unusual was happening:

1) They had a low platelet count - Platelets are the blood cells that plug holes in our blood vessels to stop us from bleeding. Platelets also activate the clotting system, but they are not typically low in people who develop blood clots.

2) They developed antibodies to a protein released by platelets (PF4) – People who had VITT developed antibodies against a protein released by platelets called PF4. This reaction caused the platelets to become very active and to increase the chance of developing abnormal blood clots. Forming antibodies against PF4 is a reaction doctors have seen before. A similar reaction happens in people who develop a rare reaction to the anticoagulant heparin (heparin-induced thrombocytopenia; HIT).

3) Their blood clots occurred in unusual places – Most blood clots not related to COVID are found within the blood vessels in legs or arteries of lungs. It is less common for people to have blood clots in large veins in their brain (cerebral venous sinus thrombosis (CVST)) or in blood vessels in their abdomen (splanchnic vein thrombosis) which is what was seen with the people in this study. There are treatments available, but blood clots found in these places do carry a high risk of bad outcomes, including death.


This Evidence Summary is based on the following article:

Greinacher A, Thiele T, Warkentin TE, et al. Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination. N Engl J Med. 2021 Apr 9. doi: 10.1056/NEJMoa2104840. PubMed

Lori-Ann Linkins
Author Details

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: Saturday, May 8, 2021


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.