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Increased risk of blood clots persists long after COVID-19

Text after the original press release written by Ola Nilsson.

Even several months after COVID-19, one is still at an increased risk of blood clot and bleeding. In the worst case, a blood clot can be fatal however, it can often be treated or prevented. These are the findings of a study from Umeå University, Sweden, where researchers analyzed all COVID-19 cases in Sweden from the start of the pandemic until May last year.
Picture: Dr Anne-Marie Fors Connolly, the head of the study (left) and Ioannis Katsoularis, one of the first authors of this study (right). Credits: Hans Karlsson and Klas Sjöberg.

Anne-Marie Fors Connolly, MIMS Clinical Research Fellow, researcher at the Department of Clinical Microbiology at Umeå University, who also works as a clinician at Norrland’s University Hospital, lead the recent study about the risk of blood clots and bleeding following COVID-19, which is now published in the British Medical Journal (BMJ).

“The results underline the importance of people getting vaccinated against COVID-19 but also of healthcare providers identifying individuals with COVID-19 who are at increased risk of blood clots so that blood thinners can be used, says Ioannis Katsoularis,” doctoral student at Umeå University and cardiologist at University Hospital of Umeå.

In the study, the researchers found that the risk of developing deep vein thrombosis (i.e. blood clots in the deep blood vessels), stayed elevated for up to three months after COVID-19. During the first month, this risk was five times higher than for people without COVID-19. Among hospitalized COVID-19 patients, the risk of bleeding was also two-fold increased.

When a blood clot lodges in the lung, a potentially fatal condition emerges, called pulmonary embolism. The researchers observed that the risk of developing pulmonary embolism stayed increased up to six months after COVID-19. In particular, the risk of pulmonary embolism during the first month was 32 times higher than in individuals without COVID-19. It was also observed that among those who were on treatment with blood thinners since before COVID-19, the risk of pulmonary embolism was 70% lower, suggesting a protective effect.

The risk of blood clots and bleeding increased with the severity of COVID-19, and was highest in patients who required intensive care.

“With a population of more than one million people affected by the disease and four million controls, this study is one of the largest of its kind and contributes to the existing body of knowledge” says Anne-Marie Fors Connolly, physician at University Hospital of Umeå and principal investigator of the study.

The study was based on data from national registers from the Swedish National Public Health Agency and the Swedish National Board of Health and Welfare. All individuals in Sweden with a positive SARS-COV-2 test from the start of the pandemic until May 2021 were included in the study. The researchers calculated the incidence of blood clots and bleeding after SARS-CoV-2 infection and compared it to the incidence of these conditions in individuals without the infection. The risk of these complications was calculated using two different methods; a matched cohort study and a self-controlled case series study.

The study is published in the British Medical Journal, BMJ.

About the scientific publication
Risks of deep vein thrombosis, pulmonary embolism and bleeding after COVID-19: nationwide self-controlled case series and matched cohort study
Ioannis Katsoularis* & Osvaldo Fonseca-Rodriguez*, Paddy Farrington, Hanna Jerndal, Erling Häggström-Lundevaller, Krister Lindmark, Anne-Marie Fors Connolly.
BMJ, The British Medical Journal
Doi: 10.1136/bmj‑2021‑069590
* Shared first author


Anne-Marie Fors Connolly
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Ioannis Katsoularis

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Original press release available in English here:
Original press release available and Swedish here:

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