By now, most people are aware of the risk of developing COVID for a long time after having COVID-19. But new research suggests that the virus may increase the risk of developing blood clots, and that risk remains higher than normal for a year afterward.
That’s the main conclusion of a large new study published in the journal Circulation. The study analyzed data from 48 million people registered with Britain’s National Health System from January 2020 to the day before vaccines against COVID-19 became available in December 2020. The researchers found 1 .4 million diagnoses of COVID-19 and, among these, 10,500 patients. who developed problems related to blood clots.
Researchers found that in the first week after someone received a diagnosis of COVID-19, 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 in the brain) was almost 22 times higher than in someone who did not have the virus. The risk dropped in the second week, but was still high: less than four times greater than in someone who did not have the virus.
For clots that occur in the veins, such as deep vein thrombosis and pulmonary embolism, the risk in the first week after being diagnosed with COVID-19 was 33 times greater. After three to four weeks after a person had been diagnosed, it was about eight times greater. The risk was still 1.8 times higher 27 to 49 weeks later compared to people who had never had COVID-19.
The risks were there regardless of how severe a person’s COVID-19 was, but were higher in those who were hospitalized with the virus. Clot risks were also higher in black and Asian patients.
Generally, clots were rare. The overall risk of developing an arterial clot within 49 weeks of being diagnosed with COVID-19 was 0.5%, and it was 0.25% for a venous clot in that time frame. (To translate this into real-world health problems, 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 problems.)
This raises many questions about safety after having COVID-19, including why this can happen and what you should watch out for. Here’s what you need to know.
Why can COVID-19 increase the risk of blood clots?
The study didn’t explore this, it simply found an association, but there are a few theories as to why this link might exist.
A big one is that the virus can cause inflammation in the body. “COVID triggers an inflammatory response that can enhance blood clotting and damage vascular structures,” says infectious disease expert Amesh A. Adalja, MD, principal investigator at the Johns Hopkins Center for Health Security. As a result, he says, “an elevated risk of clotting may persist.”
It’s also possible that an increased risk of blood clots is simply how the virus works, says Thomas Russo, MD, professor and chief of infectious diseases at the University of Buffalo in New York. “It seems to be part of the pathophysiology of this virus,” he says.
Dr. Russo says doctors have known “since 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 multiple organs” from blood clots, he says.
Can this happen with other infections?
Doctors say other infections can cause a greater than usual risk of blood clots. Dr. Adalja notes that the link is “well described” with shingles causing an increased risk of heart attacks.
“These clots have also been described with influenza,” says Dr. Russo. “However, they are more common with COVID.”
Signs of a blood clot
It is normal for blood to clot under certain circumstances, such as when you have a cut. But blood clots can be a problem when they form and create a blockage or travel to other areas of the body, such as the lungs or brain, according to Medline Plus. Symptoms of dangerous blood clots depend on where they are in the body. For Medline Plus, they may include:
- A stomachache
- Sudden or gradual pain in the arm, along with swelling, tenderness and warmth
- shortness of breath
- Pain with deep breathing
- Fast breathing
- Increased heart rate
- Problems to speak
- Vision problems
- Weakness on one side of the body
- A sudden and severe headache
- Chest pain
- shortness of breath
- Pain in left arm
How to stay safe from blood clots if you’ve had COVID-19
Dr. Adalja emphasizes that this risk is rare. However, Dr. Russo says it’s still a good idea to at least be aware of the risk of blood clots and what symptoms they might look like.
It’s also a good idea to get your full series of COVID-19 shots, including any boosters you’re eligible for, whether you’ve had COVID or not, Dr. Russo says. “That’s one more reason to try to protect yourself from contracting COVID,” he says.
Korin Miller is a freelance writer specializing in general wellness, health and sex, and lifestyle trends, with work appearing in Men’s Health, Women’s Health, Self, Glamor and more. He has a master’s degree from American University, lives on the beach and hopes to own a teacup pig and a taco truck one day.