Up to six months after infection, Covid-19 increases the risk of developing serious blood clots, according to a study published Thursday in the British Medical Journal (BMJ).
Work by Swedish researchers reveals an increased risk of deep vein thrombosis (a blood clot in the leg) up to three months after infection with Covid-19, pulmonary embolism (a blood clot in the lungs) until six months later and a bleeding event up to two months later.
This risk is higher in patients with comorbidities and those with severe Covid-19.
It was more marked during the first pandemic wave compared to the second and third waves, also underlines the study.
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We already knew that Covid-19 increases the risk of serious blood clots (known as venous thrombosis), but we had less information on how long this risk was increased and whether it varied during the different epidemic waves.
More significant thrombosis during the first wave
To conduct their study, the researchers identified more than one million people in Sweden who were infected with SARS-Cov2 between February 1, 2020 and May 25, 2021, matched by age, sex and place of residence to more than four million people who had not tested positive for Covid.
They then calculated the rates of deep vein thrombosis, pulmonary embolism and bleeding in people who had Covid-19 during a control period and made comparisons with a control group.
According to the researchers, the increased risks observed during the first wave compared to the following two could be explained by the subsequent improvements in treatments and vaccination coverage in older patients.
Prevent the formation of clots
For the researchers, these results justify the taking of measures aimed at preventing thrombotic events (such as the administration of treatments which prevent the formation of a clot in the blood vessels), in particular for patients at high risk, and reinforce the importance of vaccination against Covid-19.
The AstraZeneca vaccine had caused thrombosis.
But the benefit “greatly” outweighed the risk, the WHO estimated in April 2021.