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Catching Covid is associated with a five-fold increase in the risk of deep vein thrombosis (DVT) and a 33-fold increase in the risk of a life-threatening blood clot on the lungs within 30 days of infection, the data shows.

The findings, published in the British Medical Journal on Thursday, could help explain the doubling of incidence and deaths from blood clots in England since the start of the pandemic compared to the same periods in 2018 and 2019.

They also help put into context the very small increased risk of blood clots associated with vaccination against Covid-19. “The degree of complications associated with Covid-19 is much stronger and lasts much longer than what we might have after vaccination,” said Dr Frederick Ho, senior lecturer in public health at the University of Glasgow, who was not involved in the research.

“Even people with mild symptoms who do not need to be hospitalized may have a small increased risk of [blood clots].”

Although previous research has suggested that catching Covid was associated with an increased risk of blood clots, it was unclear how long this risk persisted and whether mild infections also increased people’s risk.

To address these uncertainties, Anne-Marie Fors Connolly from Umeå University in Sweden and her colleagues measured the risk of DVT, pulmonary embolism – a blood clot on the lung – and various types of bleeding, such as gastrointestinal bleeding or a burst blood vessel in the brain, in over a million people with confirmed Covid infections and over 4 million uninfected individuals.

Overall, they identified a 33-fold increase in the risk of pulmonary embolism, a five-fold increase in the risk of DVT, and an almost two-fold increase in the risk of bleeding within 30 days of infection. People remained at increased risk of pulmonary embolism for six months after being infected, and for two and three months for bleeding and DVT.

Although the risks were highest in patients with more severe disease, even those with mild Covid had a threefold increased risk of DVT and a sevenfold increased risk of pulmonary embolism. No increased risk of bleeding was found in those who suffered mild infections.

“Pulmonary embolism can be fatal, so it’s important to be aware [of this risk]”, Connolly said. “If you suddenly find yourself short of breath and it doesn’t pass, [and] you have been infected with the coronavirus then it might be a good idea to seek help as we see this risk increasing for up to six months.

Ho said the findings remained relevant even in the Omicron era because current vaccines were highly effective against severe Covid, but breakthrough infections were common even after a third dose of a vaccine.

“Despite the potential for worrying new variants, most governments are removing restrictions and focusing on figuring out how best to live with Covid. This study recalls the need to remain vigilant in the face of complications associated with even a mild Sars-CoV-2 infection, in particular [blood clots].”

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