New observational research out of Sweden has tracked more than one million COVID-19 cases for months after their acute illness in order to determine how the disease influences subsequent risk of blood clots. The findings indicate COVID-19 significantly increases a person’s risk of deep vein thrombosis and pulmonary embolism in the months after infection.
The large study, published in The BMJ, looked at health data from 1,057,174 positive COVID-19 cases. This covers every single positive recorded case in Sweden from the beginning of the pandemic up to May 2021. A control group of four million age- and sex-matched subjects not positive with COVID was generated to quantify the increase in risk for blood clots caused by infection.
In the 30 days after an initial COVID-19 diagnosis the researchers found a 33-fold increase in pulmonary embolism risk, a five-fold increase in deep vein thrombosis (DVT) risk and a two-fold increase in general bleeding events. The heightened risk, compared to the uninfected control group, persisted 60 days for bleeding, 90 days for DVT, and 180 days for pulmonary embolism.
Factoring in acute disease severity, the researchers found those initially hit hardest with COVID-19 did experience greater long-term risk for blood clotting events. However, the increased risk was not zero in those experiencing mild COVID. Those with only a mild infection still showed a three-fold risk of DVT and a seven-fold risk of pulmonary embolism.
While this new study is the most robust to investigate the relationship between blood clots and SARS-CoV-2 infection it is not the first to identify this association. A large UK study last year found similar signs when it compared rates of hematological and vascular events after COVID-19 to rates of those same events after COVID vaccination.
That study more generally found significant increases in rates of stroke, heart attack and blood clotting a month after a COVID infection. It also found these rates of adverse events were much higher after COVID-19 than after vaccination, validating the suggestion that the deleterious after-effects of COVID-19 are much more impactful than any potential harms caused by vaccination.
Another UK study published last year found incidences of and deaths from thromboembolism doubled in the first six months of the pandemic compared to rates seen in the years prior.
The new study relied on data collected both before and after the widespread rollout of vaccinations so the study didn't differentiate between vaccination status. As a result there is no direct data on what protective effect is conferred by vaccination in regards to the increased blood clotting risk after SARS-CoV-2 infection. An editorial in The BMJ accompanying the new study, from researchers at the University of Glasgow, hypothesizes there still may be a risk of these blood clotting events in patients experiencing vaccine breakthrough infections.
“… current vaccines are highly effective against severe covid-19 but confer only moderate protection against infection with the Omicron variant,” the editorial states. “Although many infections with the Omicron variant are mild, the new study confirms an increased risk of venous thromboembolism even among those with milder infections who do not require admission to hospital.”
Ioannis Katsoularis, an author on the new Swedish study from Umeå University, said the findings affirm the value of vaccination in reducing the likelihood and severity of COVID-19. Plus, he said older COVID-19 patients who are already at a high risk of blood clotting events should be closely watched in the months after an acute infection.
“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,” Katsoularis added.
The new study was published in The BMJ.
Source: Umeå University
