COVID and Blood Clots: What’s the Risk?
Key takeaways:
While a COVID-19 infection can cause common symptoms of fever, cough, and shortness of breath, it can also cause blood clots in some people.
The COVID virus affects blood vessels by causing inflammation. This can lead vessels to form blood clots.
The best way to prevent blood clots from COVID is to avoid infection. You can prevent COVID by keeping your vaccinations up to date and using a high-quality mask in crowded areas.
Table of contents
Most people think of COVID-19 as a respiratory illness. But a COVID infection can cause much more than a cough and sore throat. The body can respond to the virus in many ways, including widespread inflammation. And sometimes this inflammation can lead to blood clots. Blood clots can cause serious problems, like heart attacks, strokes, and even death.
How does COVID affect blood vessels?
The COVID virus affects blood vessels by triggering high levels of inflammation in the body. Researchers think the virus can cause the immune system to become overactive. A series of reactions leads to damaged cells and lots of inflammation. This causes blood to become sticky and can trigger the body to create blood clots.
The risk of blood clots is highest during the first week of COVID infection. But the risk doesn’t go away after that first week. There’s evidence that the risk of blood clots remains high long after COVID symptoms have passed. And this may explain why some people develop long COVID.
Search and compare options
In fact, a large analysis of health data from UK Biobank looked at adults who got COVID in 2020 (before vaccination was available). The analysis followed this group for almost 3 years, until October 2022. The researchers compared this group of 10,000 adults who tested positive for the original strain of COVID against more than 200,000 adults who didn’t get COVID in 2020.
They found that, over the course of the study, the risk of heart attack, stroke, and death from any cause was more than twice as likely in people who’d tested positive for COVID than it was for people who hadn’t had COVID. This is relevant because the main cause of heart attack and stroke is blood clots.
Where can COVID blood clots form?
COVID can affect both small blood vessels (capillaries) and large ones (veins and arteries), all over the body.
Blood clots in the large veins and arteries can lead to:
Deep vein thrombosis (DVT)
Pulmonary embolism (blood clot in the lung)
Blood clots in the lungs and legs are the most common, but heart attacks and strokes can also happen. And smaller blood clots can happen in almost every other organ of the body too, including the kidneys, liver, and even your toes.
How common are COVID blood clots?
Clots due to COVID are rare. A large study of 48 million adults with COVID found the risk of developing a blood clot only increased by 0.25%. But the risk of blood clots is higher if you have other risk factors (more on this below).
How COVID-19 increases the risk of heart attacks and stroke: New evidence shows that blood vessel inflammation caused by the original strain of COVID lasts longer than previously thought.
Would you know if you had a blood clot on your lungs? Find out what a pulmonary embolism feels like, and if you might be at risk.
COVID vaccination is here to stay. Here’s what you need to know about staying current with your COVID vaccine.
Quiz: Do I have DVT?
What are the symptoms of a blood clot?
The symptoms of a blood clot depend on which body part is affected and how big the blood vessels are that are affected.
With small blood clots, you may have no apparent symptoms at all.
With larger blood clots, symptoms might include:
Chest pain and shortness of breath, with a blood clot in the heart or lungs
Leg or arm pain and swelling, if the blood clot is one of your limbs
Sudden numbness or weakness in the face, arm, or leg, if the blood clot causes a stroke
Loss of vision or trouble speaking, with certain types of strokes
If you have any of these symptoms, seek medical attention right away. Blood clots can be serious and life-threatening in some cases.
How serious are blood clots from COVID?
Blood clots can be serious because they can affect breathing and circulation. Clots from a COVID infection are more serious in people who are critically ill and hospitalized. This may be because they're confined to bed. Or it may be due to the body’s extreme inflammatory response to the virus.
In severe cases, blood clots can lead to death. It’s not clear how often people with mild COVID symptoms develop clots. Serious blood clots may be less likely because people can continue to move around and stay active.
How to test for blood clots after COVID
Imaging tests like ultrasound scans, CT scans, and MRIs can pick up large clots. But these tests won’t be able to diagnose blood clots in small blood vessels.
Sometimes, lab work can help. And many times, suspected blood clots may be diagnosed by an experienced healthcare professional based on a person’s symptoms.
What are some risk factors for developing COVID blood clots?
There are many different risk factors for getting blood clots from COVID. You’re at greater risk if you're hospitalized with COVID. In fact, up to 30% of people in the intensive care unit (ICU) with COVID develop blood clots. Others at risk include people who have:
Heart disease
Diabetes
Blood disorders, like sickle cell disease and thalassemia
A history of blood clots
Cancer
How do you treat blood clots?
Treatment for blood clots often involves blood thinners (anticoagulants). While they don’t make the clot go away immediately, they help to prevent it from getting bigger. And they also help to prevent new blood clots from forming. With treatment, the blood clot will dissolve and disappear over time.
Some common blood thinners include:
Enoxaparin (Lovenox)
Warfarin (Coumadin)
Rivaroxaban (Xarelto)
Apixaban (Eliquis)
In severe cases, healthcare professionals may recommend other treatments or procedures to immediately remove the clot. These treatments can have serious side effects and are only used in extreme cases.
Can you prevent COVID blood clots?
In some cases, you may be able to prevent blood clots by staying as active as possible. Try to avoid staying in one position for more than an hour.
Studies have shown better outcomes when people in the hospital get treatment with blood thinners. And this may be especially useful for people already at high risk for forming blood clots.
Here are some more tips to try and prevent blood clots:
Raise your feet and legs when sitting or lying down.
Avoid putting pillows under your knees.
Wear loose-fitting clothing, especially socks or stockings.
Can the COVID vaccine cause blood clots?
Blood clots from the COVID vaccine are extremely rare. There were a few reports of blood clots early on after the Johnson & Johnson vaccine. They happened in about 4 out of every 1 million people who were vaccinated. This vaccine is no longer available.
Even though there’s a very small risk of blood clot with the vaccine, it’s still very safe and the best way of preventing COVID and the risk of long-term complications from COVID — including blood clots. In fact, the risk of developing blood clots from COVID infection is much higher than the risk from vaccination. Put another way, the benefits of vaccination for COVID far outweigh any risk of the vaccine.
The bottom line
It’s well known now that a COVID infection can cause widespread inflammation in your body, including in blood vessels. This increases the risk of developing blood clots — in all types of blood vessels, all over the body.
If you’re otherwise healthy, your risk is pretty low. But you have a much higher risk if you have a chronic health problem.
The risk of a blood clot is highest when you first get COVID, but it can remain high for years after. If you have COVID and risk factors for a blood clot, talk to your primary care provider about steps you can take to lower your risk. And seek medical attention right away if you notice symptoms.
Why trust our experts?
References
Agency for Healthcare Research and Quality. (2017). Your guide to preventing and treating blood clots.
Centers for Disease Control and Prevention. (n.d.). COVID-19.
Centers for Disease Control and Prevention. (2024). Coronavirus disease 2019 (COVID-19) vaccine safety.
Centers for Disease Control and Prevention. (2024). Understanding blood clots infographic.
Hilser, J. R., et al. (2024). COVID-19 is a coronary artery disease risk equivalent and exhibits a genetic interaction with ABO blood type. Arteriosclerosis, Thrombosis, and Vascular Biology.
Klok, F. A., et al. (2020). Incidence of thrombotic complications in critically ill ICU patients with COVID-19. Thrombosis Research.
Knight, R., et al. (2022). Association of COVID-19 with major arterial and venous thrombotic diseases: A population-wide cohort study of 48 million adults in England and Wales. Circulation.
Ludmann, P. (2024). COVID toes and other rashes COVID-19 may cause. American Academy of Dermatology Association.
Merschel, M., et al. (2022). Blood clot risk remains elevated nearly a year after COVID-19. American Heart Association News.
National Heart, Lung, and Blood Institute. (2022). Deep vein thrombosis (DVT).
Paranjpe, I., et al. (2020). Association treatment dose anticoagulation with in-hospital survival among hospitalized patients with COVID-19. Journal of the American College of Cardiology.
Pellicori, P., et al. (2021). What type of heart and blood vessel problems complicate COVID-19 infections, how common are they and what other medical conditions do these patients have? Cochrane Evidence Synthesis and Methods.
Xie, Y., et al. (2022). Long-term cardiovascular outcomes of COVID-19. Nature.
