PEOPLE who recovered from a Covid infection during the first wave of the pandemic remained at risk of potentially life-threatening blood clots for nearly a year, according to scientists.

The findings, based on an analysis of 48 million health records, 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 alone during 2020, although the excess risk to individuals remains small and reduces over time. 

The research – which included input from scientists at Edinburgh University - shows that people with only mild or moderate disease were also affected.

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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, before Covid vaccines were rolled out, to compare the risk of blood clots after Covid-19 with the risk at other times. 

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 four weeks.

The researchers also studied conditions caused by blood clots in the veins, including 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 the infection.

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The new study shows that there was also an effect on people whose Covid-19 did not lead to hospitalisation, although their excess risk lower.

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 two men in 100 infected may have a stroke or heart attack after Covid-19 infection.

As well as pre-dating mass Covid vaccination, however, the data also covers a period before the emergence of the more virulent Delta strain and, more recently, the Omicron variant and its sublineages.  

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.

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.”

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Jonathan Sterne, Professor of Medical Statistics and Epidemiology at the University of Bristol 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.”

Co-lead Angela Wood, Professor of Biostatistics at the University of Cambridge and an associate director of the British Heart Foundation Data Science Centre, added: “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.”

Funding for the research came from the British Heart Foundation (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.