Study confirms rare blood clotting risk linked to ChAdOx1-S vaccine - benefits still outweigh risks

Pharmaceutical and biotechnology company AstraZeneca had recently come under fire for rare blood clotting events — causing a temporary suspension of the vaccine in multiple countries. In April, the European Medicines Agency concluded the vaccine is potentially linked to blood clotting in people with low blood platelets who received the dose. However, they emphasized unusual blood clotting was a rare side effect.

New research published in BMJ confirmed evidence of blood clotting finding a small risk after receiving one Oxford-AstraZeneca ChAdOx1-S severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine.

The authors write:

“We did, however, observe an increased rate of venous thromboembolic events, corresponding to 11 excess venous thromboembolic events per 100000 vaccinations and including a clearly increased rate of cerebral venous thrombosis with 7 observed events versus 0.3 expected events among the 282572 vaccine recipients.”

The study findings could help with limiting the vaccine to people with a lower risk of blood clotting. Some countries such as Canada and the United Kingdom had already restricted vaccine use to older age groups.

Reviewing the data

The researchers investigated the likelihood of blood clotting events for 282,572 people in Denmark and Norway. Using data from healthcare registries, they reviewed information on people aged 18-65 who received their first dose of the ChAdOx1-S vaccine from February 9, 2021, to March 11, 2021.

The health information they collected also included hospitalization stays or outpatient clinic contacts such as an emergency room visit. They specifically looked for cardiovascular and hemostatic diagnoses such as arterial events, venous thromboembolism, thrombocytopenia/coagulation disorders, and unusual bleeding.

In total, the researchers studied the health information of 281,264 people eligible for the study. About 148,792 people were from Denmark, and 132,472 people from Norway. Both groups were predominately made up of women. About 26.4% of the vaccinated group had health data available less than 28 days after vaccination.

Blood clotting risk after vaccination

About 83 arterial events were found with a morbidity ratio of 0.97. Of the 83, the researchers observed increases in intracerebral hemorrhages. This was estimated to occur 1.7 for every 100,000 vaccinations.

The team expected 30 venous thromboembolic events, but they found a higher number — 59 with a morbidity ratio of 1.97. About 11 excess events were estimated to occur for every 100,000 vaccinations. There was also a small increased risk for other health issues, including pulmonary embolism, lower limb venous thrombosis, and other venous thromboses.

There were 2.5 events of blood clotting in the brain for every 100,000 vaccinations. The morbidity ratio was estimated to be 20.25 after observing 7 instances of cerebral venous thrombosis.

The morbidity ratio for any type of thrombocytopenia/coagulation disorder was 1.52, which corresponded with 3 excess events for every 100,000 vaccinations. About 5.1 excess bleeding events were observed for every 100,000 vaccinations. This included bleeding from the respiratory tract and any other unspecified bleeding.

Of the entire vaccinated cohort, there were 15 deaths — this was lower than the 44 deaths expected to occur in the general population.

About 0.24% or 643 people tested positive for COVID-19 infection after vaccination. In the 18-65 age group in Denmark, the researchers found less than 5 cerebral venous thrombosis events 28 days after testing positive for COVID-19. In contrast, there was no cerebral venous thrombosis observed in 18-65-year-olds with a positive test result in Norway.

Risk factors for clotting events

There were some age differences for venous thromboembolism. There was a slightly higher instance of venous thromboembolism in the 18-44 year age group compared to people in the 45-65 year age group.

There were almost no differences in blood clotting based on gender. However, the researchers noticed an increased risk for thrombocytopenia/coagulation disorders in men 14 days after vaccination. However, this might have been due to another variable. Excluding hospital contacts of less than 5 hours erased the observed increase in thrombocytopenia/coagulation disorders.

“Our study provides evidence of an excess rate of venous thromboembolism, including cerebral venous thrombosis, among recipients of the OxfordAstraZeneca covid-19 vaccine ChAdOx1-S within 28 days of the first dose. The absolute risks of these events were, however, small.”

The researchers conclude that blood clotting is a very rare side-effect and the benefits of vaccination continue to outweigh the risks.

Journal reference:
  • Pottegård A, et al. Arterial events, venous thromboembolism, thrombocytopenia, and bleeding after vaccination with Oxford-AstraZeneca ChAdOx1-S in Denmark and Norway: population based cohort study. BMJ, 2021. doi:,
Jocelyn Solis-Moreira

Written by

Jocelyn Solis-Moreira

Jocelyn Solis-Moreira graduated with a Bachelor's in Integrative Neuroscience, where she then pursued graduate research looking at the long-term effects of adolescent binge drinking on the brain's neurochemistry in adulthood.


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