Studies Show Slightly Elevated Risk of Blood Clots Following COVID-19 Vaccine


The results of 2 analyses in the United Kingdom show a small increase in the potential for intracranial thrombosis after vaccination.

There is a slightly elevated risk of intracranial thrombosis following vaccination with the ChAdOx1-S (AstraZeneca) COVID-19 vaccine, the results of 2 studies published in PLOS Medicine showed.

In the first study from the University of Edinburg, investigators analyzed electronic health records (EHRs) of about 26 million individuals in England. The second study from the same university focuses on a dataset of approximately 11 million individuals in England, Scotland, and Wales.

Cases of thromboses, which occur when blood clots block an artery or vein, have been reported after individuals were received the COVID-19 vaccine.

However, the rates of common arterial and venous events, including deep vein thrombosis, myocardial infarction, pulmonary embolism, and stroke, are normally hard to measure based on just care reports.

In the first study, investigators analyzed the EHRs of individuals living in England, and approximately 21 million of the individuals were vaccinated sometime during the study time span between December 2020 and March 2021.

The investigators found that for individuals ages 70 years or older, the risk of arterial and venous thrombotic events was slightly lower in the 28 days following vaccination with either the Pfizer BNT162b2 vaccine or the ChAdOx1-S vaccine. They adjusted for the range of demographic characteristics, as well as comorbidities.

Additionally, in individuals under aged 70 years, the risk of arterial and venous thrombotic events was comparable in the 28 days following vaccination. However, a small increased rate of intracranial venous thrombosis was observed after vaccination from ChAdOx1-S.

“In adults under 70 years, the small increased risks of intracranial venous thrombosis and hospitalization with thrombocytopenia after first vaccination with ChAdOx1-S are likely to be outweighed by the vaccines’ effect in reducing COVID-19 mortality and morbidity,” investigators said in a statement.

There was a corresponding excess risk of an estimated 0.9 to 3 per million, varying by age and sex, which was approximately twice the rate of those who were unvaccinated, even after adjusting for a range of demographic characteristics and comorbidities.

Investigators did not see the same effect after vaccinations with the BNT162b2 vaccine.

In the second study, investigators linked data from December 2020 through June 2021 from multiple sources, which included mortality, primary and secondary care, and virological testing.

They compared the rates of cerebral venous sinus thrombosis (CVST) events, which is a rare blood clot in the brain, in the 90 days prior to vaccination and 4 weeks following the first dose of either vaccine.

Investigators found that there was a small, elevated risk of CVST events following vaccination with ChAdOx1-S, which is equivalent to 1 additional event per 4 million individuals vaccinated. That is twice as high as before vaccination.

The study results showed no association between the BNT162b2 vaccine and CVST.

Investigators said they plan to conduct future studies to include other vaccines, as well as second and booster vaccinations.


Two studies find only small elevated risk of blood clots following AstraZeneca COVID-19 vaccination. EurekAlert. News release. February 22, 2022. Accessed February 23, 2022.

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