Blood clotting is a well-recognized complication of SARS-CoV-2 infection. It has also been identified as an extremely rare side effect of certain COVID-19 vaccines. The risk with vaccines is exceedingly low and individuals are at a significantly higher risk of developing a blood clot from COVID-19 infection than following COVID-19 vaccination.
Yazan Abou-Ismail, MD, a hematologist and assistant professor in the Division of Hematology & Hematologic Malignancies at University of Utah Health, specializes in disorders of clotting and bleeding and answers questions about the association of blood clots with COVID-19 and certain vaccines.
Blood clots can occur either in the veins or arteries. Deep venous thrombosis (DVT) occurs when a blood clot forms in the deep veins of your body, usually in the lower leg, thigh, or pelvis. A blood clot that travels from the deep veins and blocks one of the arteries in your lungs is called a pulmonary embolism (PE) and may be life-threatening. Less commonly, venous blood clots can also occur in other locations, such as the brain or abdomen.
In contrast, arterial blood clots occur in the vessels that supply blood to organs, such as the heart or brain, causing conditions like a heart attack or stroke. Blood clots can be life-threatening but are preventable and treatable if discovered early. According to the Centers for Disease Control and Prevention (CDC), about 900,000 people experience a DVT or PE and 100,000 people die from blood clots in the U.S. each year.
The Moderna or Pfizer-BioNTech mRNA vaccines have not been shown to increase the risk of blood clots in multiple studies. However, two COVID-19 vaccines are associated with an extremely rare syndrome known as vaccine-induced thrombotic thrombocytopenia (VITT). It was discovered in March 2021 in connection to the AstraZeneca COVID-19 vaccine and then later with the Johnson & Johnson COVID-19 vaccine. In rare cases—2 to 20 per million—antibodies that the body produces as a side effect of the vaccine lead to uncontrolled activation of platelets. This causes both low platelet counts and blood clots to form.
The occurrence of this rare antibody is a random event without a known association with a person’s medical history or any particular conditions. “There’s no way to predict who’s likely to develop the rare antibody that triggers VITT,” Abou-Ismail says. “While it is a little more common in younger women compared to other populations, it remains exceedingly rare even in that group.”
