Why COVID-19 could be causing blood clots—and what you can do to lower your risk


Editor’s note: As what we know about COVID-19 evolves, so could the information contained in this story. Find our most recent COVID-19 blog posts here, and learn the latest in COVID-19 prevention at the Centers for Disease Control and Prevention.

As if the breathing complications associated with COVID-19 aren’t worrisome enough, doctors are discovering another risk posed by the coronavirus: blood clots that can lead to life-threatening strokes, heart attacks and pulmonary embolism. 

As COVID-19 traveled across Europe and hit hard in New York City, word began to spread of patients riddled with clots in their brain, hearts, lungs and legs—and sometimes all over. In Los Angeles, doctors had to amputate the right leg of a Broadway star because of severe clotting. Medical staff at The Ohio State University Wexner Medical Center began seeing blood clots in some of their COVID-19 patients too. 

 “It’s very scary for a patient and it’s alarming for a medical center too,” said Danielle Blais, PharmD, a specialty practice pharmacist in cardiology at the Ohio State Richard M. Ross Heart Hospital. “We called in experts from so many different disciplines to figure out how best to treat these patients, and we continue to learn more every day.”

Blood clots are a serious condition: Untreated, they can cause damage to your brain, heart and lungs. Death or long-term complications are a real concern.

While the health care community is still learning the ways COVID-19 attacks the body, it appears that a few factors are causing the increased risk of clots, said Matthew Exline, MD, medical director of the medical intensive care unit at the Ohio State Wexner Medical Center.

Blood clotting factors 

First, COVID-19 can cause severe inflammation, which can trigger your clotting system. 

“When you, say, fall and skin your knee, it turns your immune system on, and one of the ways your immune system reacts to an injury is by making your clotting system more active,” Exline said. “It kind of makes sense that your body would say, if I see an infection, I need to be ready to clot. But when the infection is as widespread and inflammatory as COVID-19, that tendency to clot can become dangerous.” 

And when you’re sick with COVID-19 or following stay-at-home or quarantine orders, you probably aren’t moving much.

“If you’re immobile, you have an increased risk factor for blood clots,” Exline said. 

Paired together, inflammation and immobility create a near perfect environment for blood clots in your legs and lungs, Exline said. Patients with severe cases of COVID-19 seem especially susceptible, as do those with other health risk factors such as cancer, obesity and a history of blood clots.

Blood clot treatment 

Knowing this, health care providers have changed the way they treat COVID-19 patients to specifically address the risk of clotting. It’s taken quick, widespread collaboration. At the Wexner Medical Center, specialty practice pharmacists along with critical care medicine, cardiology, hematology, emergency medicine and internal medicine doctors developed guidelines on how to manage these patients, Blais said. 

“We’ve done the amount of work that some people would take a year or two to put together in a matter of weeks,” she said.

Now, patients who are sick enough from COVID-19 to go to the hospital receive blood tests to gauge the activity of their clotting systems. Recent studies have demonstrated that patients with COVID-19 are prone to clotting, but patients in the ICU may also be at risk for bleeding.

"Health care providers must carefully weigh the risks and benefits of anticoagulation for each individual patient," Exline says.

Those whose clotting systems aren’t particularly active receive treatments to prevent clots such as compression socks, inflatable cushions for their calves or small injections of blood thinners. Those with more active clotting systems receive full doses of blood thinners if they’re not at a high bleeding risk.

“We’re having to be thoughtful about our approach with treatment, especially because there is limited data in COVID-19 patients,” said Tiffany Ortman, PharmD, a specialty practice pharmacist in outpatient care at the Ross Heart Hospital. 

After patients leave the hospital, health care providers continue to monitor patients for clotting symptoms and lower their risk through medications. Some currently active research studies are attempting to understand how long patients should stay on anticoagulation medication as they recover from COVID-19.  

Blood clots and COVID-19 vaccination

Recent news from both Europe and the United States has raised concerns about blood clots after COVID-19 vaccination. It's important to note that the most common vaccines in the U.S. — the Pfizer and Moderna vaccines — have not been found to have a high risk of blood clots.

Blood clots have been reported in Europe after the AstraZeneca vaccine and in the U.S. with the Johnson & Johnson vaccine, but these incidents have been extremely rare, Exline notes. 

"There may be an extremely low risk of blood clots with one type of COVID-19 vaccine, but you're more at risk for injury driving to your vaccine appointment than from any side effect from the vaccine itself," Exline says. "It's important to remember that the risk of blood clots from a COVID-19 infection is much more likely than any side effect of a vaccine. If you want to protect yourself from blood clots, get vaccinated."

Who's most at risk for blood clots, and what to look for

While those with severe cases of COVID-19 appear to be more affected by blood clots, those who don’t come to the hospital could still be at risk, said Aaron Dush, PharmD, a specialty practice pharmacist at the James Cancer Hospital and Solove Research Institute. 

They, as everyone, should monitor for signs of clots and possible stroke or heart attack: 

  • facial drooping 
  • weakness of one arm or leg
  • difficulty speaking 
  • new swelling, tenderness, pain or discoloration in the arms or legs 
  • sudden shortness of breath 
  • chest pain or pain radiating to the neck, arms, jaw or back

Call 911 if you’re experiencing concerning symptoms.

And Dush’s biggest recommendation for those with COVID-19 at home: Keep moving. Stay hydrated. When you are seated, try to keep your legs elevated. 

Keep the blood, quite literally, flowing.