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Increased blood clot risk can persist for six months after Covid, study finds


blood clot covid


An increased risk of developing a serious blood clot persists for six months after a Covid infection, a study has suggested.

The analysis of national registry data from more than a million Swedish people with confirmed Covid infection between the start of the pandemic and May 2021 found an increased risk of pulmonary embolism for up to six months and deep vein thrombosis up to three months.

They also found higher risk of bleeding events for two months after infection.

Reporting in the BMJ the researchers also said there was a higher risk in those with co-morbidities, patients with more severe Covid-19 and also that the risk had been higher in the first waves rather than the second or third waves.

The data showed a five-fold increased risk of deep vein thrombosis, a 33-fold increase in risk of pulmonary embolism, and an almost twofold increase in risk of bleeding in the first 30 days after infection.

  • A first deep vein thrombosis occurred in 401 patients with Covid-19 and 267 control patients.
  • A first pulmonary embolism event occurred in 1,761 patients with Covid-19 and 171 controls
  • A first bleeding event occurred in 1,002 patients with Covid-19 and 1,292 control patients

Increased rates of deep vein thrombosis and pulmonary embolism were also found among mild, non-hospitalised covid-19 patients, the researchers noted.

‘Our findings arguably support thromboprophylaxis to avoid thrombotic events, especially for high-risk patients, and strengthen the importance of vaccination against Covid-19’, the researchers concluded.

In a linked editorial, researchers at the University of Glasgow said while many infections with the Omicron variant are mild, the findings that blood clot risk is apparent even in those with less severe disease, this group may account for a substantial number of thromboembolic events simply because of the high level of infections.

‘Despite the potential for new variants of concern, most governments are removing restrictions and shifting their focus to determining how best to “live with covid”.

‘[This]study reminds us of the need to remain vigilant to the complications associated with even mild SARS-CoV-2 infection, including thromboembolism,’ they concluded.

During last year’s Covid vaccination programme, all UK patients aged under 40 were offered Pfizer or Moderna vaccines instead of Astra Zeneca following concerns over rare blood clots.

However, a large UK study concluded that Covid infection carried a substantially higher and longer risk of blood clots and other related adverse events than Covid vaccines, including Astra Zeneca.