Covid-19 patients are at increased risk of deep vein thrombosis up to three months after infection and pulmonary embolism up to six months, according to a Swedish study published in the BMJ.

While it is well-known that Covid-19 increases the risk of serious blood clots, less evidence exists on the length of time this risk is increased, if risk changed through pandemic waves, and whether the virus increases the risk of major bleeding. 

To better understand these uncertainties, a team of researchers in Sweden used national registries to identify more than one million people with Covid and then looked at the rates of deep vein thrombosis and pulmonary embolism, as well as the risk of major bleeding.

Covid patients had a 33-fold increase in risk of pulmonary embolism

The researchers then performed two analyses: firstly, they calculated the rates of deep vein thrombosis, pulmonary embolism, and bleeding in Covid-19 individuals during a control period (before and long after infection) and compared it to the rates in different time intervals after a positive Covid test result.

In the second analysis, they calculated the rates of deep vein thrombosis, pulmonary embolism, and bleeding during the period 1-30 days after Covid-19 diagnosis and compared them to the corresponding rates in the control group.

They found that compared with the control period, the risk of deep vein thrombosis was significantly increased for 90 days after infection, 180 days for pulmonary embolism and 60 days for bleeding.

After taking into account potentially influential factors, they found a fivefold increase in risk of deep vein thrombosis, a 33-fold increase in risk of pulmonary embolism, and an almost twofold increase in risk of bleeding in the 30 days after infection.

Risks were highest in patients with more severe Covid-19 and during the first pandemic wave compared with the second and third waves, which the researchers say could be explained by improvements in treatment and vaccine coverage in older patients after the first wave.

However, even patients with mild Covid were at increased risk of deep vein thrombosis and pulmonary embolism, but they were not at increased risk of bleeding.

The findings support measures to prevent thrombotic events in Covid patients

While the study cannot establish a cause, the researchers say the findings were largely consistent after further analyses and are in line with similar studies on the association between Covid-19 and thromboembolic events.

For this reason, the authors say the study's findings support measures to prevent thrombotic events, especially for high risk patients, and strengthen the importance of vaccination against Covid.

In a linked editorial, researchers at the University of Glasgow say the findings also serve as a reminder to "remain vigilant" to the complications associated with Covid-19, particularly as governments remove restrictions and continue to push us into a world in which we learn to "live with Covid".