An increased risk of potentially life-threatening blood clots lasts for at least a year after Covid-19 infection, according to a UK study looking at unvaccinated adults from the first wave of the pandemic.

It suggests that Covid-19 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, the researchers said.

People with only mild or moderate disease are also affected, although, they stressed that the excess risk to individuals remains small and reduces over time.

Health records from across England and Wales were used to compare the risk of blood clots after Covid-19 with the risk at other times.

It showed that in the first week after a Covid-19 diagnosis, people were 21 times more likely to have a heart attack or stroke. This dropped to 3.9 times more likely after four weeks.

The risk of blood clots deep vein thrombosis and pulmonary embolism was 33 times greater in the first week after a Covid-19 diagnosis, dropping to eight times higher risk after four weeks, they reported in the journal Circulation.

A higher risk of blood clots after Covid-19 remained for the duration of the study, 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.

Putting the results in context, the researchers said in those 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.

The data used in the study was before mass vaccination rollout in the UK, and before more recent variants but they team are now looking to understand the risk of blood clots in relation to vaccines and widespread infection with Delta and Omicron.

Study co-lead Professor Jonathan Sterne, Professor of Medical Statistics and Epidemiology at the University of Bristol, 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-author Dr William Whiteley, clinical epidemiologist and neurologist at the University of Edinburgh, added:  ‘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.’

Previously, the Oxford/Astra Zeneca Covid vaccine was found to be associated with increased risk of thrombocytopenia, venous thromboembolism, and other rare arterial thrombotic events.

However, Covid infection carries a substantially higher and longer risk of blood clots and other related adverse events than either Oxford/Astra Zeneca or Pfizer/BioNTech vaccination, a large UK study later concluded.