Ailments that can linger long after COVID have been well documented, and the list appears to be lengthening.

Now scientists are adding blood clots to the list, some so severe that they lead to death. Even a mild case of COVID can put one at risk in the weeks following infection, according to a new report released Monday.

The London-based study, published in British medical journal Heart, examined the records of nearly 20,000 U.K. residents who had experienced COVID, and compared them with those of similar individuals who had not.

Researchers found that individuals who had been diagnosed with COVID but weren’t hospitalized were still at a risk nearly three times as high for venous thromboembolism—blood clots in veins that can include deep vein thrombosis and pulmonary embolism—when compared with similar individuals who hadn’t had COVID.

Such individuals were more than 10 times as likely to experience death, researchers found.

“Given the high population exposure to COVID-19, these reports may herald a significant imminent public health problem,” the authors wrote.

A wider range of adverse events, and a greater incidence of them, was found among those who had been hospitalized with COVID. Such individuals were at a nearly 28-fold greater risk for venous thromboembolism, a nearly 22-fold greater risk for heart failure, and a nearly 18-fold greater risk for stroke, when compared with peers who hadn’t experienced COVID.

Most adverse events noted in the study occurred within 30 days of infection, but “the risk remains augmented for a prolonged period thereafter,” according to the authors.

The study examined records of patients during the first two waves of COVID in the United Kingdom. Further study is needed to determine how long the heightened risk for blood clots and other adverse cardiovascular events lasts, and to determine what might be done to help patients at high risk for such events, the authors wrote.

COVID is known to be linked to an increased risk of blood clots and related issues, including heart attack and stroke, immediately after diagnosis. But just how long that elevated risk lasts is unknown. 

A September study published in the American Heart Association journal Circulation found that the risk of deep vein thrombosis was nearly double in those who had experienced COVID when compared with those who had not. It also found that an elevated risk of deep vein thrombosis and arterial thrombosis persisted for nearly a year after infection. It examined the anonymously collected data of nearly every adult in England and Wales.