News Release

PHILADELPHIA – Individuals hospitalized with COVID-19 were more likely to develop venous thromboembolism – a potentially life-threatening condition – than those hospitalized with influenza, according to a new study from the Perelman School of Medicine at the University of Pennsylvania. The study, published in the Journal of the American Medical Association, evaluated the absolute risk of hospitalized deep venous thrombosis or pulmonary embolism events within 90 days after admission. “Our research found this association both before and during vaccine availability, showing that the risk was not stemming from vaccination,” said Vincent Lo Re III, MD, MSCE, an associate professor of Infectious Diseases and Epidemiology at Penn and the study’s first author. “And that’s particularly important because individuals say they do not want to receive a COVID-19 vaccination because of clotting risks. COVID-19 itself is the true risk of these dangerous clots, not the vaccines.”

Venous thromboembolism, which is comprised of both deep vein thrombosis and pulmonary embolism, begins when a blood clot forms in a vein deep within the body, often the leg or pelvic region. The condition is dangerous because the clot can block blood circulation. During arterial thromboembolism, a clot causes a sudden interruption of blood flow to an organ or body part which in turn can lead to tissue damage, heart attack, or stroke.

Although COVID-19 is primarily considered a respiratory illness, some evidence suggests the virus may also induce excessive blood clotting, or hypercoagulability, in the body, but early studies on the topic were smaller and inconclusive.

This Penn study was the largest yet to tackle the issue, comprising of over 90,000 total patients. Among people hospitalized with influenza, the 90-day absolute risk of developing venous thromboembolism was 5.3 percent. For those hospitalized with COVID-19, 90-day risk was 9.5 percent before vaccine availability and 10.9 percent after COVID-19 vaccines became available. The 90-day risk of arterial thromboembolism was 14.4 percent in patients hospitalized with influenza compared with 15.8 percent in those with COVID-19 before vaccine availability and 16.3 percent during vaccine availability.

Lo Re and colleagues said one potential explanation for the increased risk may be the coronavirus’ ability to infect endothelial cells, which can incite inflammation and abnormalities in the coagulation process. However, the research team says more research is needed to confirm the association and investigate potential causes and possible mechanisms behind the blood conditions.

With collaboration from colleagues in Canada and Europe, the Penn researchers plan on examining COVID-19-related thrombotic cases outside the hospital setting and via an international meta-analysis.

This study was supported by the United States Food and Drug Administration (75F40119D10037, 75F40119F19001).


Penn Medicine is one of the world’s leading academic medical centers, dedicated to the related missions of medical education, biomedical research, excellence in patient care, and community service. The organization consists of the University of Pennsylvania Health System and Penn’s Raymond and Ruth Perelman School of Medicine, founded in 1765 as the nation’s first medical school.

The Perelman School of Medicine is consistently among the nation's top recipients of funding from the National Institutes of Health, with $550 million awarded in the 2022 fiscal year. Home to a proud history of “firsts” in medicine, Penn Medicine teams have pioneered discoveries and innovations that have shaped modern medicine, including recent breakthroughs such as CAR T cell therapy for cancer and the mRNA technology used in COVID-19 vaccines.

The University of Pennsylvania Health System’s patient care facilities stretch from the Susquehanna River in Pennsylvania to the New Jersey shore. These include the Hospital of the University of Pennsylvania, Penn Presbyterian Medical Center, Chester County Hospital, Lancaster General Health, Penn Medicine Princeton Health, and Pennsylvania Hospital—the nation’s first hospital, founded in 1751. Additional facilities and enterprises include Good Shepherd Penn Partners, Penn Medicine at Home, Lancaster Behavioral Health Hospital, and Princeton House Behavioral Health, among others.

Penn Medicine is an $11.1 billion enterprise powered by more than 49,000 talented faculty and staff.

Share This Page: