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 hospitalized with COVID-19. The new study shows that there was also an effect on people whose COVID-19 did not lead to hospitalization, although their excess risk was not as great as for those who had severe disease and were hospitalized.

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 analyzed 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."

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 and strategies to prevent vascular events will be important as we continue through the pandemic."

Angela Wood, Professor of Biostatistics at the University of Cambridge, Associate Director of the British Heart Foundation Data Science Centre, and study co-lead

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."