COVID-19 infection increases risk of serious blood clots 3 to 6 months later: Study

Being infected with COVID-19 raises the risk of developing serious blood clots, a new study suggests.

An international team of researchers from Sweden, the United Kingdom and Finland compared more than 1 million people in Sweden with a confirmed case of the virus between February 2020 and May 2021 to 4 million control patients who tested negative.

They found three to six months after contracting COVID-19, patients were at increased risk of being diagnosed with blood clots in their legs or lungs, according to results published in the journal BMJ on Wednesday.

Specifically, patients had a significantly increased risk of deep vein thrombosis, a blood clot that forms deep in the thigh or the lower leg, up to three months after a COVID-19 infection.

Patients also had a heightened risk of developing a pulmonary embolism, a clot that develops in a blood vessel and travels to a lung artery, up to six months after having the virus.

The team said its results add to a growing body of evidence about the link between COVID-19 and serious blood clots, while adding new information about how long the risk might last.

"The present findings have major policy implications," the authors wrote, adding that the report "strengthens the importance of vaccination against COVID-19."

They also said the findings suggest that COVID-19 patients -- "especially high-risk patients" -- should take anticoagulation medicine, which are medications to help prevent these clots.

During the course of the study period, the team saw 401 cases of DVT among the COVID-19 patients, compared to 267 cases among the negative patients.

Meanwhile, there were 1,761 cases of PE among virus patients in comparison with 171 cases among the control patients.

COVID-19 patients were at higher risk of blood clots if they had underlying conditions, had a severe case of the virus or if they were infected during the first wave of the pandemic in early 2020.

However, there wasn't just a risk of blood clots. The study also found an increased risk of any kind of bleeding up to two months after a COVID-19 infection.

The team noted there were limitations, including that the study was observational rather than a randomized controlled trial.

Additionally, the researchers recognized that clotting in COVID-19 patients may be underdiagnosed and information about patients' vaccination status was not available.

Despite the risk of blood clots following COVID-19 infections being well-documented, it's unknown what biological mechanisms are at play. However, there are theories.

One study from Michigan Medicine and the U.S. National Heart, Lung and Blood Institute suggested "rogue" antibodies from a COVID-19 infection cause blood cells to lose their anti-clotting properties.

Another study from Yale School of Medicine suggested specific proteins are produced by endothelial cells -- cells that line blood vessels -- due to inflammation from the virus and lead to blood clots.

"It remains to be established whether SARS-CoV-2 infection increases the risk of venous thromboembolism or bleeding more than it does for respiratory infections, such as influenza, but also whether the period of [anticoagulation medicine] after COVID-19 should be extended," the authors wrote.

Dr. Raffaele Macri contributed to this report.