Covid linked to increased risk of serious blood clots for up to six months after infection

Scientists believe new research could support offering some coronavirus patients drugs to prevent clots, Andy Gregory reports

A doctor observes a blood clot expelled by a Covid-19 patient in a Mexico City hospital (Hector Vivas/Getty Images)

Coronavirus can increase the risk of serious blood clots for up to six months after infection, a new study suggests.

Researchers from Umea University analysed data for more than one million people in Sweden who tested positive for coronavirus in the 15 months to May last year, comparing it to that of more than four million people without the virus.

In findings published in the British Medical Journal, the scientists discovered the risk of pulmonary embolism – a blocked blood vessel in the lungs – was “significantly increased” for six months after infection.

This was also true for deep vein thrombosis (DVT) in the three months after infection, with a heightened risk of bleeding also observed for two months.

Those with underlying health problems and those who suffered more severe coronavirus infections were more at risk, the study found.

With the risk of serious blood clots found to be higher during the first wave of the pandemic, compared to the second and third, the researchers suggested that this could reflect the role of vaccines and treatments, particularly for older people.

The researchers concluded that their findings “arguably support thromboprophylaxis [preventative treatment] to avoid thrombotic events, especially for high risk patients, and strengthen the importance of vaccination against Covid-19”.

The study also looked at the risk of clots in the period after coronavirus symptoms began, compared to long before people tested positive, and long after their symptoms disappeared.

The results showed a 33-fold increase in risk of pulmonary embolism, a five-fold increase in risk of DVT, and an almost two-fold increase in risk of bleeding in the 30 days after infection.

In absolute terms, this means a first DVT occurred in 401 patients with coronavirus (an absolute risk of 0.04 per cent) compared to 267 patients without (absolute risk 0.01 per cent) over this time period.

Meanwhile, a first pulmonary embolism occurred in 1,761 patients with coronavirus (absolute risk 0.17 per cent) and 171 without (absolute risk 0.004 per cent).

A first bleed occurred in 1,002 patients with coronavirus (absolute risk 0.1 per cent) and 1,292 without (absolute risk 0.04 per cent).

Risks continued for most patients for up to a period of six months.

It comes as UK experts warned that – despite high levels of vaccination – rising numbers of coronavirus infections in older adults could see an increase in hospitalisations and deaths.

Infection rates are at an all-time high in England, Imperial College London’s latest React-1 study found, echoing findings last week in the Office for National Statistics which suggested that a record one in 13 people in the UK had the virus in the week ending 26 March.

With the two Omicron variants, BA.1 and BA.2, having caused twin peaks in the pandemic – one in January and another in March – the number of people in hospital with coronavirus in the UK has now climbed to its highest level for more than 13 months.

Government figures show there were 20,396 patients in hospital with coronavirus on Tuesday, however 56 per cent were not being treated primarily for coronavirus – up from around a quarter last summer and autumn.

Additional reporting by PA