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Risk of blood clots remains for almost a year after COVID-19 infection, study suggests

Red blood cells

Infographic for Circulation paper on the increased risk of blood clots after COVID-19 infection

Infographic for Circulation paper on the increased risk of blood clots after COVID-19 infection

Press release issued: 20 September 2022

COVID-19 infection increases the risk of potentially life-threatening blood clots for at least 49 weeks, according to a new study of health records of 48 million unvaccinated adults from the first wave of the pandemic.

The findings suggest that the COVID-19 pandemic may have led to an additional 10,500 cases of heart attacks, strokes and other blood clot complications such as deep vein thrombosis in England and Wales in 2020 alone, although the excess risk to individuals remains small and reduces over time.

The research – involving a large team of researchers led by the Universities of Bristol, Cambridge, and Edinburgh, and Swansea University – shows that people with only mild or moderate disease were also affected. The authors suggest that preventive strategies, such as giving high-risk patients medication to lower blood pressure, could help reduce cases of serious clots.

Researchers 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.

In the first week after a COVID-19 diagnosis, people were 21 times more likely to have a heart attack or stroke, conditions which are mainly caused by blood clots blocking arteries. This dropped to 3.9 times more likely after 4 weeks.

The researchers also studied conditions caused by blood clots in the veins: these include deep vein thrombosis and pulmonary embolism – a clot in the lungs that can be fatal. 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 higher risk of blood clots after COVID-19 remained for the study duration, although by 26 to 49 weeks it had dropped to 1.3 times more likely for clots in the arteries and 1.8 times more likely for clots in the veins.

Most previous research studied the impact of COVID-19 on blood clotting in people hospitalised with COVID-19. The new study shows that there was also an effect on people whose COVID-19 did not lead to hospitalisation, although their excess risk was not as great as for those who had severe disease and were hospitalised.

The authors say that the risk of blood clots to individuals remains low. In people at the highest risk – men over the age of 80 – an extra 2 men in 100 infected may have a stroke or heart attack after COVID-19 infection.

The data analysed was collected in 2020, before the mass vaccination rollout in the UK, and before more recent COVID-19 variants such as Delta and Omicron were widespread. The researchers are now studying data beyond 2020 to understand the effect of vaccination and the impact of newer variants.

The research is published in the journal Circulation and was supported by the BHF Data Science Centre at Health Data Research UK, the Longitudinal Health and Wellbeing COVID-19 National Core Study, Data and Connectivity National Core Study and the CONVALESCENCE study of long COVID.

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 hospitalised 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 and 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.”


‘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 [open access]

Further information

About the BHF Data Science Centre
The British Heart Foundation Data Science Centre is a partnership between Health Data Research UK (HDR UK) and the British Heart Foundation (BHF). We work closely with patients, the public, NHS organisations, researchers, and clinicians to promote the safe and ethical use of data for research into the causes, prevention and treatment of all diseases of the heart and circulation.

Our vision is to improve the public’s cardiovascular health through the power of large-scale data and advanced analytics across the UK. Funded by the BHF and embedded within HDR UK, the Centre provides the leadership, co-ordination and engagement needed to deliver this vision, through building capability, capacity and infrastructure to drive excellence in data-enabled cardiovascular research.

To find out more about the BHF Data Science Centre, visit, email or follow us on Twitter @BHFDataScience

About the National Institute for Health and Care Research Bristol Biomedical Research Centre (NIHR Bristol BRC)
The National Institute for Health and Care Research Bristol Biomedical Research Centre’s (NIHR Bristol BRC) innovative biomedical research takes science from the laboratory bench or computer and develops it into new drugs, treatments or health advice. Its world-leading scientists work on many aspects of health, from the role played by individual genes and proteins to analysing large collections of data on hundreds of thousands of people. Bristol BRC is unique among the NIHR’s 20 BRCs across England, thanks to its expertise in ground-breaking population health research.

About the COVID-19 Longitudinal Health and Wellbeing National Core Study

  • The COVID-19 Longitudinal Health and Wellbeing National Core Study (LHW NCS ) is one of six national core studies established by Sir Patrick Vallance, Government Chief Scientific Adviser, to support and accelerate the UK’s research response to COVID-19.
  • The LHW NCS are using health data to identify and respond to essential questions to rapidly inform policy, operations and planning and maintain resilience against COVID-19 across the UK.
  • The UKRI funded LHW NCS aims to understand the health, social and economic impacts of the COVID-19 pandemic by uniting UK wide established population cohorts  and national anonymised electronic health records  to inform policy. 
  • The Study has a large, UK wide collaborative, multidisciplinary team including UCL (project lead) and the Universities of Bristol, Oxford, Edinburgh, Glasgow, Swansea, Cambridge, York, Ulster, Essex, West of England, King’s College London, the London School for Hygiene and Tropical Medicine and Bradford Institute for Health Research and the National Institute for Health and Care Excellence (NICE)
  • The LHW NCS research themes include: UK Longitudinal Linkage Collaboration, Serology, Vaccination, Healthcare Disruption, Society and Health, Mental Health and Long COVID.
  • The Long COVID theme (CONVALESCENCE Study) is a NIHR-UKRI funded study that will use data from established population cohorts and national anonymised electronic health records to achieve the following aims: How can we best define Long COVID? What are its risk factors and mechanistic pathways? What are the consequences for physical and mental health, and for work, education and social and familial relations? Can we enhance diagnosis and management through GP records? Convalescence is a multi-disciplinary collaboration between many universities and the National Institute for Health and Care Excellence (NICE). The CONVALESCENCE research themes include: Long COVID definition, risk, outcomes and methods to improve Long COVID diagnosis and management. With new data being collected for Deep Phenotyping and longitudinal qualitative sub-studies.

About SAIL Databank
SAIL Databank is a rich and trusted population data repository based within Swansea University Medical School. It contains billions of person-based records relating to health and administrative data, some dating back more than a quarter of a century. SAIL Databank is accredited to the highest international standards for an information management system (ISO27001). It exists to make discoveries and develop policies that improve lives by providing approved researchers with secure, linkable and anonymised data that can be accessed and analysed from anywhere in the world.

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