Illustration of a blood clot in a vein to represent the risks of venous thromboembolism blood clots

Findings from a large study led by the University of California San Francisco (UCSF) show that the risk of experiencing venous thromboembolism after COVID-19 are low in those not admitted to hospital.

Writing in the journal JAMA Network Open, the team showed that while overall risks were low, individuals who were over 55 years, male, who had a history of blood clotting problems, or who had a body mass index (BMI) of over 30 were at the highest risk.

Venous thromboembolism, where blood clots form in the veins, is a known complication of COVID-19. However, the majority of studies noting this adverse effect have been carried out in hospitalized or seriously ill COVID-19 patients.

To investigate the risk in people with less severe COVID-19, Margaret Fang, a professor at the UCSF School of Medicine, and colleagues carried out a study including 398,530 patients infected with SARS-CoV-2 between Jan 2020 and Jan 2021.

The average age of the participants was 44 years, 54% were women and 54% were of Hispanic ethnicity. None were admitted to hospital for COVID-19. The researchers assessed the risk of blood clots in the veins in the 30 days after infection and also in the 30 days after that.

Overall, 292 cases of venous thromboembolism occurred in the study group. Overall risks were low, but higher in the first 30 days after infection than later at a rate of 0.58 vs 0.09 per 100 person years, in the 30 days after infection vs later than 30 days, respectively.

Risk of experiencing a blood clot went up with age, with significantly higher risks in those aged 55 years or older. Men were also at increased risk (around 50% increase in relative risk), as were those who had experienced prior venous thromboembolism or thrombophilia (2.5-7.5 fold increase in risk). Obesity also increased the risk for a clot (50% increase in relative risk or more).

“In this cohort study of outpatient adults with COVID-19, we found that although the absolute risk of venous thromboembolism was low overall, selected patient characteristics were associated with higher rates,” conclude the authors, “particularly in the first 30 days after COVID-19 diagnosis.”

“These findings may help identify subsets of patients with COVID-19 who could benefit from venous thromboembolism preventive strategies and more intensive short-term surveillance.”

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