Blood Clotting, COVID-19 and Vaccines


Author: Kylene Metzger

University of Utah

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 obstruct blood flow. Blood clots are often triggered by certain events such as injuries, surgeries, or infections. Since the beginning of the COVID-19 pandemic, blood clotting has been identified as a side effect of SARS-CoV-2 and later as an extremely rare side effect of some COVID-19 vaccines.

Yazan Abou-Ismail, MD, a hematologist at University of Utah Health, answered questions about the association of blood clots with COVID-19 and certain vaccines.

Where do blood clots occur?

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). Both medical conditions may be life-threatening, but are preventable and treatable if discovered early. In the U.S. each year, about 900,000 people experience a blood clot and 100,000 people die from blood clots.

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 moderate illness have also developed blood clots. Data from the beginning of the pandemic indicated that the incidence of blood clots in severe COVID-19 illness ranged from 20 to 40 percent. The incidence in people with mild or moderate COVID-19 illness was three to nine percent.

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

Patients who develop blood clots as a result of COVID-19 infection usually have severe COVID-19 illness (hospitalized in the ICU), respiratory failure, require high amounts of oxygen, or have other underlying illnesses. Other underlying risks for blood clots include patients with a previous medical history of blood clots or a hereditary blood clotting disorder. Those who have hypertension, diabetes, are obese, or have cancer are also at higher risk.

What is the association between blood clotting and certain COVID-19 vaccines?

Some 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, 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 in unusual areas. VITT is not associated with the Moderna or Pfizer-BioNTech mRNA vaccines.

According to the CDC, as of June 30, 2021, there have been 35 confirmed reports of VITT in connection to the J&J vaccine out of more than 12 million doses that have been administered in the U.S. In the United Kingdom, there are more than 300 confirmed cases of VITT connected to the AstraZeneca vaccine. Although VITT is extremely rare, it is a serious and treatable condition. Doctors now know how to recognize and treat VITT, and patients who receive the J&J vaccine are informed how to recognize its side effects.

What are symptoms of VITT?

Symptoms of VITT include:

  • Severe or persistent headaches
  • Chest pain
  • Trouble breathing
  • Leg swelling/pain
  • Severe abdominal pain
  • Confusion
  • Vision changes
  • Seizures

Anyone experiencing these symptoms within four weeks of vaccination should contact their health care provider or seek immediate medical attention.

What data backs up the fact that VITT is an extremely rare side effect?

Out of more than 12.5 million people who received the J&J vaccine in the U.S., 35 cases have been reported to the CDC. That’s an incidence of two to three cases per million. Both men and women can get VITT but women between the ages of 20 to 50 are at a higher risk of experiencing VITT from the J&J vaccine. In this group, the incidence is about 10 per million, which is also still extremely rare.

The benefits of vaccination outweigh these rare potential risks. The CDC has estimated that every million doses prevent between 60 to 140 deaths and 400 to 1,000 hospitalizations from COVID-19.

Is it possible to experience blood clots months after having COVID-19 or getting a COVID-19 vaccine?

Based on current information available about VITT, there is no evidence that blood clotting could occur more than four weeks after getting vaccinated. However, blood clotting is possible months after having COVID-19 for patients who have had long hospitalizations, had surgery, are immobilized, or have a history of underlying health conditions such as obesity, diabetes, or blood clots. This is variable based on the patient and length of illness.

What do you tell your patients?

Whenever something like VITT happens, it causes the public to worry. We need to remember that COVID-19 vaccines are very effective at protecting against COVID-19. What I tell my patients is you have to weigh the risks and benefits. The risk of developing serious side effects from COVID-19 vaccines is extremely unlikely. Getting infected with COVID-19 has a much higher risk of complications, such as blood clots, long-term symptoms, and even death.