Can COVID-19 increase the risk of developing deep vein thrombosis?

Deep vein thrombosis occurs when a blood clot (or is also called a thrombus) forms in one or more veins deep in the body, usually in the legs. Deep vein thrombosis can cause leg pain or swelling, but it can also occur without symptoms. This disorder is caused by different factors, and now there is more evidence than show that when a person is infected with the coronavirus increases the risk of deep vein thrombosis by five times.

The main causes of deep vein thrombosis are damage to a vein in surgery or trauma and inflammation from infections or injuries. With the pandemic, COVID-19 was added as one of the risk factors that can trigger this thrombosis.

The finding was made through an investigation in Sweden. It was also found that COVID-19 increases the risk of a life-threatening blood clot in the lung by 33 times in the 30 days after infection, data suggest.

The results were published in the specialized journal British Medical Journal (BMJ). They help explain the increase in the incidence and deaths from blood clots since the start of the pandemic in different countries.

They also help to contextualize the very small increase in the risk of blood clots associated with vaccination against Covid-19. “The degree of complications associated with Covid-19 is much stronger and lasts much longer than we might be receiving after vaccination,” said Dr. Frederick Ho, professor of public health at the University of Glasgow, who was not involved in the research, but made an editorial comment in the same journal.

The sequelae of COVID-19 can not only develop in people with severe conditions in the acute phase of infection. “Even people with mild symptoms who don't need to be hospitalized may have a small increased risk of [blood clots],” Dr. Ho warned.

Although previous research had suggested that contracting COVID-19 was associated with an increased risk of blood clots, it was not clear how long this risk was maintained, or whether mild infections also increased people's risk.

To resolve these uncertainties, Anne-Marie Fors Connolly, from the University of Umeå, in Sweden, and her colleagues measured the risk of deep vein thrombosis, pulmonary embolism -a blood clot in the lung- and various types of bleeding, such as gastrointestinal bleeding or rupture of a blood vessel in the brain, in more than one million people who had confirmed the diagnosis of COVID-19 by testing, and in more than 4 million uninfected individuals.

Following the research, scientists identified a 33-fold increase in the risk of pulmonary embolism, a five-fold increase in the risk of deep vein thrombosis (DVT), and an almost two-fold increase in the risk of bleeding within 30 days of infection. The risk of pulmonary embolism was maintained for six months after infection, and that of hemorrhage and DVT for two and three months.

Although the risks were higher in patients with a more severe disease, even those with mild COVID-19 had a three times higher risk of deep vein thrombosis and seven times greater risk of pulmonary embolism. No increased risk of bleeding was found in those who experienced mild infections.

“Pulmonary embolism can be life-threatening, so it's important to be aware [of this risk],” Connolly said. “If you suddenly find yourself short of breath, and you don't miss it, [and] you have been infected with the coronavirus, then it might be an idea to seek help, because we found this risk increased up to six months.”

Dr. Ho argued that the results were still relevant even at this time of the pandemic with the predominance of the OMICRON variant of concern, since current vaccines were very effective against severe COVID-19, but infections in immunized people are common, even after a third dose of a vaccine.

A previous study, which had been conducted in England, revealed that the incidence and mortality from thromboembolism had doubled since the start of the pandemic in 2020, compared to the same periods in 2018 and 2019. The same study reported comparable increases among individuals without COVID-19.

“Despite the potential for new variants of concern, most governments are removing restrictions and changing their approach to determining the best way to live with COVID-19. This study reminds us of the need to remain vigilant for complications associated even with mild SARS-CoV-2 virus infection, including blood clots,” Dr. Ho warned in his editorial in BMJ magazine.

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