People infected with Covid-19 are at an increased risk of deep vein thrombosis — a blood clot in the leg — up to three months, pulmonary embolism — a blood clot in the lung — up to six months, and a bleeding event up to two months, finds a study.

The findings, published by The BMJ, also showed a higher risk of events in patients with underlying conditions, and those with more severe Covid.

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Researchers from Umea University in Sweden said these results support measures to prevent thrombotic events (thromboprophylaxis), especially for high risk patients, and strengthen the importance of vaccination against covid-19.

It is well known that Covid increases the risk of serious blood clots (known as venous thromboembolism or VTE), but less evidence exists on the length of time this risk is increased, if risk changed during the pandemic waves, and whether Covid also increases the risk of major bleeding.

To address these uncertainties, researchers set out to measure the risk of deep vein thrombosis, pulmonary embolism, and bleeding after covid-19.

For the study, the team identified more than one million people with confirmed Covid infection between February 1, 2020 and May 25, 2021, and matched them with more than four million people who had not had a positive SARS-CoV-2 test result.

The researchers found a five-fold increase in risk of deep vein thrombosis, a 33-fold increase in risk of pulmonary embolism, and an almost two-fold increase in risk of bleeding in the 30 days after infection.

Risks were the highest in patients with more severe Covid and during the first pandemic wave compared with the second and third waves, which the researchers say could be explained by improvements in treatment and vaccine coverage in older patients after the first wave.

Even among mild, non-hospitalised Covid patients, the researchers found increased risks of deep vein thrombosis and pulmonary embolism. No increased risk of bleeding was found in mild cases, but a noticeable increase was observed in more severe cases.

“This is an observational study, so the researchers cannot establish the cause,” researchers said.

“Our findings arguably support thromboprophylaxis to avoid thrombotic events, especially for high risk patients, and strengthen the importance of vaccination against covid-19,” they said.