An increased risk of blood clots persists for close to a year after a COVID-19 infection, a large study shows.
The health records of 48 million unvaccinated adults in the United Kingdom suggest that the pandemic's first wave in 2020 may have led to an additional 10,500 cases of heart attack, stroke and other blood clot complications such as deep vein thrombosis, in England and Wales alone.
The risk of blood clots continues for at least 49 weeks after infection, the study found.
"We have shown that even people who were not hospitalized faced a higher risk of blood clots in the first wave," said study co-leader Angela Wood, associate director of the British Heart Foundation Data Science Centre.
"While the risk to individuals remains small, the effect on the public's health could be substantial and strategies to prevent vascular events will be important as we continue through the pandemic," Wood said in a news release from Health Data Research UK, which sponsors the center.
Researchers found that the risks did lessen over time.
Patients were 21 times more likely to have a heart attack or stroke in the week after their COVID diagnosis. After four weeks, the risk was 3.9 times greater than usual.
Heart attacks and strokes are mainly caused by blood clots blocking arteries.
The risk of clots in veins was 33 times greater in the week after COVID diagnosis, dropping to eight times greater after four weeks. Conditions caused by these clots include deep vein thrombosis and pulmonary embolism, which can be fatal.
By 26 to 49 weeks after a COVID diagnosis, the risk dropped to 1.3 times more likely for clots in arteries and 1.8 times more likely for clots in veins, the study showed.
While people who were not hospitalized had a lower risk, it was not zero, the study found.
Overall, individual risk remains low, the authors said. Men over 80 years of age are at highest risk.
"We are reassured that the risk drops quite quickly -- particularly for heart attacks and strokes -- but the finding that it remains elevated for some time highlights the longer-term effects of COVID-19 that we are only beginning to understand," said study co-leader Jonathan Sterne, director of the NIHR Bristol Biomedical Research Center and of Health Data Research UK South West.
The authors said steps such as giving high-risk patients blood pressure-lowering medication could help reduce cases of serious clots.
Researchers are now studying newer data to understand how vaccination and the impact of new COVID variants may affect blood clotting risks.
The findings were recently published in the journal Circulation.
More information
The U.S. Centers for Disease Control and Prevention has more on blood clots.
SOURCE: Health Data Research UK, news release, Sept. 20, 2022
