The risk of blood clots remains for almost a year after COVID-19 infection

COVID-19 infection increases the risk of potentially life-threatening blood clots for at least 49 weeks.


Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induces a prothrombotic state, but the long-term effects of COVID-19 on the incidence of vascular diseases are unclear. In a new study by a large team of researchers led by the Universities of Bristol, Cambridge, Edinburgh, and Swansea University, COVID-19 infection increases the risk of potentially life-threatening blood clots for at least 49 weeks. It also led to an additional 10,500 heart attacks, strokes, and other blood clot complications, such as deep vein thrombosis in England and Wales in 2020 alone. However, the excess risk to individuals remains small and reduces over time.

Scientists studied de-identified electronic health records across the whole population of England and Wales from January to December 2020 to compare the risk of blood clots after COVID-19 with the risk at other times. Data were accessed securely and safely via the NHS Digital Trusted Research Environment for England and the SAIL Databank for Wales.

People were 21 times more likely to experience a heart attack or stroke in the first week following a COVID-19 diagnosis. These diseases are primarily brought on by blood clots obstructing arteries. After four weeks, this became 3.9 times less common.

Scientists also looked at the conditions such as deep vein thrombosis and pulmonary embolism, a potentially lethal blood clot in the lungs. The risk of blood clots in the veins was 33 times greater in the first week after a COVID-19 diagnosis. This dropped to eight times higher risk after four weeks.

The greater risk of blood clots following COVID-19 persisted throughout the study; however, by 26 to 49 weeks, it had decreased to 1.3 times more likely for clots in the arteries and 1.8 times more likely for clots in the veins.

Most of the previous studies examined how COVID-19 affected persons who were hospitalized with the virus’ effects on blood coagulation. The latest research demonstrates that there was also an impact on individuals whose COVID-19 did not result in hospitalisation. However, their elevated risk was not as high as that of individuals with severe illness who required hospitalisation.

Authors noted, “the risk of blood clots to individuals remains low. In people at the highest risk – men over 80 – an extra 2 men in 100 infected may have a stroke or heart attack after COVID-19 infection.”

Jonathan Sterne, Professor of Medical Statistics and Epidemiology at the University of Bristol, Director of the NIHR Bristol Biomedical Research Centre, and Director of Health Data Research UK South West, who co-led the study, said: “We are reassured that the risk drops quite quickly –, particularly for heart attacks and strokes – but the finding that it remains elevated for some time highlights the longer-term effects of COVID-19 that we are only beginning to understand.”

Angela Wood, Professor of Biostatistics at the University of Cambridge, Associate Director of the British Heart Foundation Data Science Centre, and study co-lead, said: “We have shown that even people who were not hospitalized faced a higher risk of blood clots in the first wave. While the risk to individuals remains small, the effect on the public’s health could be substantial. Strategies to prevent vascular events will be important as we continue through the pandemic.”

Dr. William Whiteley, Clinical Epidemiologist, and Neurologist at the University of Edinburgh, who co-led the study, said: “The effect that coronavirus infection has on the risk of conditions linked to blood clots is poorly studied, and evidence-based ways to prevent these conditions after infection will be key to reducing the pandemic’s effects on patients.”

Journal Reference:

  1. Association of COVID-19 with major arterial and venous thrombotic diseases: a population-wide cohort study of 48 million adults in England and Wales by Jonathan A.C. Sterne et al. in Circulation. DOI: 10.1161/CIRCULATIONAHA.122.060785