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A blood clot is a collection of blood cells and coagulation proteins that clump together, forming a gel-like substance in the blood system that can block blood flow. Blood clots are often triggered by certain events such as injuries, surgeries, or infections like COVID-19.

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  at �鶹ѧ����Ʒ��, specializes in disorders of clotting and bleeding and answers questions about the association of blood clots with COVID-19 and certain vaccines.

Where do blood clots occur?

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  (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.

What is the link between blood clots and COVID-19?

The association between COVID-19 and blood clots was recognized early in the pandemic among hospitalized COVID-19 patients. These patients experienced blood clots both in deep veins and arteries, which sometimes led to strokes and heart attacks. Although these conditions have mostly been seen in patients with severe COVID-19 illness, people with non-severe illness have also developed blood clots.

Data from the beginning of the pandemic indicated that the incidence of blood clots in COVID-19 illness ranged anywhere from 5 to 40 percent, varying with individual risk factors and severity of illness.

Do certain people have a higher risk of developing blood clots if they have COVID-19?

Those at higher risk of developing blood clots as a result of COVID-19 infection include patients who:

• Have a previous medical history of blood clots or a hereditary blood clotting disorder
• Have severe COVID-19 illness (hospitalized in the ICU)
• Experience respiratory failure or require high amounts of oxygen
• Have prolonged immobility
• Have underlying illnesses such as hypertension, diabetes, obesity, or cancer

When is a person likely to develop a blood clot following COVID-19 infection?

Usually in the first week or month following infection. It’s less likely for blood clots to occur after that, but clots may occur in those with persistent COVID-19 illness or complications.

Has the incidence of blood clots from COVID-19 changed?

The incidence of blood clots seems to be trending downwards compared to earlier in the pandemic. Abou-Ismail says this may be related to many reasons such as:

• Improved testing
• Earlier recognition
• Improved COVID-19 treatment
• Better access to therapeutic approaches

Are there any treatments to prevent blood clots in COVID-19 patients?

New guidelines from the National Institutes of Health and the American Society of Hematology recommend that patients hospitalized with COVID-19 who have oxygen requirements and are not in the ICU (also referred to as “moderate COVID-19”) receive a full dose of anticoagulation therapy to help reduce blood clots that may be triggered by SARS-CoV-2 infection. Patients in the ICU are recommended to receive a prophylactic dose—a lower dose of anticoagulation or blood thinners.

“At the beginning of the pandemic, we didn’t know the best way to prevent blood clots from COVID-19 or whether the bleeding risk caused by blood thinners outweighed the benefit of blood clot prevention,” Abou-Ismail says. “Now, after having data from several large clinical trials, we have guidance on how to best prevent blood clots caused by COVID-19 without causing significant harm in hospitalized patients.”

When a patient leaves the hospital, providers can now determine individual risk of blood clots. If that individual risk is high, Abou-Ismail says some patients may benefit from temporarily continuing the use of blood thinners at home.

Do COVID-19 vaccines cause blood clots?

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  (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.”

If deciding whether a COVID-19 vaccine is worth the risk, Abou-Ismail tells his patients that ”mRNA COVID-19 vaccines are safe in patients with a history of blood clots or increased risk factors. The risk of developing a blood clot from COVID-19 infection is far higher than getting a blood clot from a COVID-19 vaccine.”

In fact, he stresses that having a history of a blood clot is even more reason to get vaccinated because these patients are more likely to get a potentially life-threatening blood clot from COVID-19 infection itself. It is in no means a reason to avoid vaccination with an mRNA vaccine.