Thromboembolism risk persists even after mild COVID‐19

COVID‐19 may be contributing to a major population burden of venous thromboembolism (VTE) according to researchers who found it was an independent risk factor for deep vein thrombosis (DVT) and pulmonary embolism for up to six months after infection.

A case control study compared data from more than one million people in Sweden who tested positive for SARS-CoV-2 up to May 2021 with more than four million control participants. It found an increased risk of DVT up to three months after COVID‐19 infection, pulmonary embolism up to six months, and a bleeding event up to two months.

The analysis, published in the BMJ, also showed a higher risk of events in patients with comorbidities, patients with more severe COVID‐19, and during the first pandemic wave compared with the second and third waves.

After adjusting for potentially confounding factors, the researchers found a fivefold increase in risk of DVT (relative incidence 4.98, 95% confidence interval 4.96 to 5.01), 33-fold increase in risk of pulmonary embolism (33.05, 32.8 to 33.3), and an almost twofold increase in risk of bleeding (1.88, 1.71 to 2.07) in the 30 days after infection.

In absolute terms, this meant that a first DVT occurred in 401 patients with COVID‐19 (absolute risk 0.04%) and 267 control patients (absolute risk 0.01%). A first pulmonary embolism event occurred in 1,761 patients with COVID‐19 (absolute risk 0.17%) and 171 control patients (absolute risk 0.004%), and a first bleeding event occurred in 1,002 patients with COVID‐19 (absolute risk 0.10%) and 1,292 control patients (absolute risk 0.04%).

The lower risks in patients during the second and subsequent pandemic waves compared with the first wave might be explained by improvements in treatment and vaccine coverage in older patients, the researchers suggested.

However the researchers noted that there were increased risks of DVT and pulmonary embolism even among mild, non-hospitalised COVID‐19 patients.

They said the results were in line with those of similar studies such as one based on 5,000 hospitalised COVID‐19 patients in the UK that found the incidence rate ratio for VTE increased about 14-fold during the first two weeks, decreasing to an increase of about eightfold during the third week and threefold during the fourth week.

“Our findings arguably support thromboprophylaxis to avoid thrombotic events, especially for high risk patients, and strengthen the importance of vaccination against COVID‐19,” they concluded.

In a linked editorial, researchers at the University of Glasgow said the findings were still relevant at a phase of the pandemic when most of the population had received COVID‐19 vaccines, because most governments are removing restrictions and shifting their focus to determining how best to “live with covid”.

They said vaccination did not offer complete protection against infection with the omicron variant, and the new study confirmed an increased risk of thromboembolism even among those with milder infections who did not require admission to hospital.

While the risk was weaker (relative incidence 5.87 for pulmonary embolism) than that among patients admitted to hospital (64.49), mild disease accounted for a much larger proportion of infections (94.5% in this study).

“This patient group may therefore contribute a substantial number of thromboembolic events,” they noted.

The study “reminds us of the need to remain vigilant to the complications associated with even mild SARS-CoV-2 infection, including thromboembolism,” they concluded.

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