Covid raises blood clot risk up to 6 months after infection, study says

Covid-19 increases the risk of developing serious blood clots up to six months after being infected, according to a study published this Thursday (6) in the British Medical Journal (BMJ).

The Swedish study reveals an increased risk of deep vein thrombosis up to three months after infection with Covid-19, of pulmonary embolism up to six months later and of a bleeding event up to two months later.

The risk was higher in patients with comorbidities and among those who suffered from the severe form of Covid-19. In addition, it was more marked during the first wave of the pandemic than the second or third, according to the study.

Covid-19 infection was known to increase the risk of serious blood clots, known as venous thrombosis, but there was less information about the period during which the risk was highest and whether it varied during different pandemic waves.

To do the study, researchers identified more than one million people in Sweden infected with Covid-19 between February 1, 2020 and May 25, 2021, and sorted them by age, sex and place of residence, with four million people who have not been infected.

With this, they calculated the rates of deep vein thrombosis, pulmonary embolism and bleeding among people who had Covid-19 over the control period and compared them with the group without contagion.

According to the researchers, the higher risks observed during the first wave compared to the two following could be explained by improvements in treatments and vaccine coverage in older patients.

For the scientists, the results justified taking measures to prevent thrombosis (such as administering treatments to prevent blood clots), particularly for high-risk patients.

They also highlighted the importance of the Covid-19 vaccine.