What You Need to Know About COVID-19, Thrombosis Risk, and Vaccination

When it comes to COVID-19 vaccines, there’s a ton of misinformation on the internet, particularly when it comes to health risks.

But recent research refutes a common anti-vaccine claim, determining that COVID-19 vaccination carries only a trivial risk of venous thromboembolism (VTE), or blood clots. The study also confirmed that the risk of blood clots is far greater from a COVID-19 infection than the vaccine.

Researchers from the University of Buffalo sought to investigate whether receiving a COVID-19 vaccine put an individual at a higher risk of VTE, a claim widely spread on social media and mainstream media. They analyzed data on more than a million adults from 2020 to 2022 and found that the excess risk of VTE was 1.4 per million vaccinated people. The results were published in February 2023 in the Journal of Clinical and Translational Science.

“Given that patients with COVID in the ICU have a 20 percent chance of developing a blood clot, the vaccine is much safer and is protective against blood clots,” says Linda Yancey, MD, an infectious diseases specialist at Memorial Hermann Hospital in Houston who was not involved in the study.

Vaccine-Induced Immune Thrombocytopenia and Thrombosis

Previous studies tied the minimal increased risk of blood clots from COVID-19 vaccines to vaccine-induced immune thrombocytopenia and thrombosis (VITT), a rare condition in which the body’s immune response to a vaccine triggers the production of an antibody called anti-platelet factor 4 (APF4). These antibodies send the body’s clotting mechanisms into overdrive and cause large or widespread blood clots.

VITT has only been associated with adenovirus vaccines, including the Johnson & Johnson COVID-19 vaccine, which was used in the United States, and the AstraZeneca vaccine, used in the United Kingdom and other countries. The condition does not occur with the mRNA COVID-19 vaccines from Pfizer and Moderna.

In 2022, the U.S. Food and Drug Administration (FDA) limited the use of Johnson & Johnson’s COVID-19 vaccine, and it is currently no longer being used in the United States, with all remaining stock having expired in May 2023.

“There is no increased risk of blood clots with the current COVID vaccines available in the United States,” says James Lawler, MD, an infectious disease specialist at Nebraska Medicine. “It is important to note that the more recent protein-component adjuvanted vaccine from Novavax also appears to have no associated increased risk of blood clots.”

Additionally, individuals who were vaccinated against COVID-19 with adenovirus vaccines in the past need not worry about blood clot risk now. “The risk was very low and was related to the period just after vaccination,” Dr. Yancey says. “At this point, that risk has faded to the background rate in the population.”

COVID-19 Infection and Blood Clot Risk

Studies have shown that COVID-19 illness carries a much higher risk of blood clots than do either of the adenovirus vaccines. A study published in 2020 in Thrombosis Research included nearly 400 patients hospitalized with COVID-19 in Italy. The researchers found that about 20 percent of those patients developed venous thromboembolism, or blood clots in the deep veins, after being hospitalized.

A study published in 2022 in The BMJ found that the risk of serious blood clots remains for up to six months after an infection with COVID-19.

When patients mention concerns about COVID-19 vaccines and blood clots, Yancey tells them: “You have a one in a million chance of getting a blood clot with the vaccine and a 1 in 5 chance of getting a blood clot with a bad case of COVID. If this were the anti-lottery, which ticket would you buy?”

The increased risk of blood clots with COVID-19 infection is higher than that associated with other serious respiratory infections, such as influenza, Dr. Lawler notes. It’s also seen in young adults, as well as the elderly. “It is one of the many complications of COVID that vaccination significantly reduces the risk for,” Lawler says.

“People with a history of blood clots are at especially increased risk for COVID and should absolutely protect themselves by remaining up to date on COVID vaccines,” he continues.

Warning Signs of Thrombosis

It’s important to know the signs of a blood clot, so that if you are affected you can seek medical help quickly. Immediate treatment drastically increases your chances of survival. Symptoms from blood clots can vary depending on the location of the blood clot. Warning signs can include:

• Leg swelling limited to one leg
• Shortness of breath 
• Chest pain
• Dizziness 
• Abdominal pain 
• Headache 
• Visual disturbance

If you have any symptoms of a blood clot, see your doctor right away. Also let them know if you have any risk factors for thrombosis, including recent COVID-19 infection. Blood clots can be dangerous and even turn fatal, but they can also be safely and effectively treated.

A Final Note on Vaccine Safety

Understandably, there’s been much confusion about the safety of COVID-19 vaccines, with so much information and misinformation circulating during the past few years. But it’s important to remember that we now have much more data on their safety than we did at the beginning of the pandemic.

“The number of COVID-19 vaccine doses is climbing towards three-quarters of a million doses in the United States, with billions more administered around the world,” says Mark Cameron, PhD, associate professor in the department of population and quantitative health sciences at Case Western Reserve University. “The safety of mRNA vaccines is well-established across multiple safety monitoring systems in the United States. That said, the CDC, FDA, and many other organizations around the world continue to conduct what’s easily considered the most intensive safety monitoring study in U.S. history.”

For the latest information on vaccine safety, he recommends reading the Centers for Disease Control and Prevention’s updated COVID-19 vaccine recommendations, as well as their COVID-19 vaccine safety landing page and information about COVID-19 vaccine safety reporting systems.

“I hope people come away with the same take-home message I have,” Dr. Cameron says, “that the benefits of COVID-19 vaccination far outweigh the rare chances of serious adverse events, especially where the same severe reactions are much more prevalent following infection with COVID-19 itself, including cardiac complications and blood clots.”