By now, most people are aware of the risk of developing long COVID after having COVID-19. But new research suggests the virus can ramp up your risk of developing blood clots—and that risk stays higher than normal for a year afterward.
That’s the main takeaway from a large new study published in the journal Circulation. The study analyzed data from 48 million people registered in Great Britain’s National Health System from January 2020 until the day before COVID-19 vaccines were made available in December 2020. The researchers found 1.4 million diagnoses of COVID-19 and, among those, 10,500 patients that developed blood clot-related issues.
The researchers discovered that, in the first week after someone received a COVID-19 diagnosis, the risk of developing an arterial blood clot (which can lead to a heart attack or stroke by blocking blood flow to the heart or brain), was nearly 22 times higher than in someone who didn’t have the virus. The risk dropped by the second week, but was still elevated—it was less than four times higher than in someone who didn’t have the virus.
For clots that happen in the veins, like deep vein thrombosis and pulmonary embolism, the risk in the first week after being diagnosed with COVID-19 was 33 times higher. After three to four weeks after a person had been diagnosed, it was about eight times higher. The risk was still 1.8 times higher between 27 and 49 weeks later when compared to people who had never had COVID-19.
The risks were there regardless of how severe a person’s COVID-19 was, but they were higher in those who were hospitalized with the virus. The clot risks were also higher in Black and Asian patients.
Overall, the clots were rare. The overall risk of developing an arterial clot in the 49 weeks after being diagnosed with COVID-19 was 0.5% and it was 0.25% for a venous clot in that time period. (To translate that into real-world health issues, it led to about 7,200 additional heart attacks or strokes and 3,500 additional cases of pulmonary embolism, deep vein thrombosis, or other vein-related health issues.)
This raises a lot of questions about safety after having COVID-19, including why this might happen and what you should be on the lookout for. Here’s what you need to know.
Why can COVID-19 raise your risk of blood clots?
The study didn’t explore this—it simply found an association—but there are some theories on why this link might exist.
A big one is that the virus can cause inflammation in your body. “COVID provokes an inflammatory response that can enhance blood clotting and damage vascular structures,” says infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. As a result, he says, “a heightened risk of clotting can persist.”
It’s also possible that an increased risk of blood clots is simply how the virus works, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. “It seems to be part of the pathophysiology of this virus,” he says.
Dr. Russo says that doctors have known “from the beginning of this pandemic that when you get infected with COVID,” you have a greater tendency to form blood clots. “Early on, we were seeing these terrible situations of people with black fingers and toes, and damage to a variety of organs” because of blood clots, he says.
Can this happen with other infections?
Doctors say that other infections can cause a higher-than-usual risk of blood clots. Dr. Adalja points out that the link is “well described” with shingles, causing an increased risk of heart attacks.
“These clots have also been described with flu,” Dr. Russo says. “However, they’re more common with COVID.”
Signs of a blood clot
It’s normal for blood to clot under certain circumstances, like when you have a cut. But blood clots can be an issue when they form and create a blockage or travel to other areas of the body, like your lungs or brain, according to Medline Plus. Symptoms of dangerous clots depend on where they’re located in the body. Per Medline Plus, they can include:
Sudden or gradual pain in your arm, along with swelling, tenderness, and warmth
Shortness of breath
Pain with deep breathing
An increased heart rate
Weakness on one side of the body
A sudden, severe headache
Shortness of breath
Pain in the left arm
How to stay safe from blood clots if you’ve had COVID-19
Dr. Adalja stresses that this risk is rare. However, Dr. Russo says it’s still a good idea to at least be aware of the risk for blood clots and what symptoms can look like.
It’s also a good idea to get your full COVID-19 vaccination series, including boosters you’re eligible for, whether you’ve had COVID or not, Dr. Russo says. “This is yet another reason to try to protect yourself from getting COVID,” he says.
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