COVID-19 Raises Risk of Lingering and Serious Blood Clotting, Swedish Study Finds

COVID-19 Raises Risk of Lingering and Serious Blood Clotting, Swedish Study Finds
Photo: Isa Foltin, Getty Images

New research from Sweden is the latest to show that people who COVID covid-19 are then more susceptible to other serious health problems down the road. The study found that COVID-19 survivors were more likely to experience blood clotting issues in the weeks and months following their infection than those not infected. These risks were much higher for people with severe COVID-19, but an increased risk remained even for those with mild illness.

The study, published in the BMJ on Wednesday, looked at the outcomes of over a million Swedish people who came down with lab-confirmed COVID-19 between February 2020 and May 2021. These residents were compared to people similarly matched in age, sex, and residency who had tested negative for the coronavirus during the same time period. The researchers were specifically looking for reported cases of deep vein thrombosis, pulmonary embolism, and bleeding.

Overall, the team found an increased risk of all three conditions in COVID-19 patients, compared to controls — one that remained even long after the infection would usually be cleared. This increased risk was apparent up to 70 days later for deep vein thrombosis, up to 110 days for pulmonary embolisms, and 60 days later for any kind of bleeding.

“The findings of this study suggest that COVID-19 is a risk factor for deep vein thrombosis, pulmonary embolism, and bleeding,” the researchers wrote.

It’s already known that SARS-CoV-2 infection can cause acute heart and cardiovascular problems, especially in severe cases. And this isn’t the first study to indicate a link between COVID-19 and clotting problems. But one advantage over past studies is that Sweden’s nationalized health care system allowed the researchers to track virtually every confirmed case of COVID-19 in the country during their study period and to easily compare their health to people without confirmed covid. Infections in general are known to be linked to clotting issues, but COVID-19 might represents a uniquely greater risk factor than the typical infectious disease, the authors say. They speculate that could be because the virus can infect blood vessel cells or due to its inflammatory effects on the immune system.

Like other studies, this research points to a clear spectrum of added risk. More than 6% of COVID-19 patients in the ICU, for instance, developed a case of pulmonary embolism, a life-threatening condition caused by blood clots in the lungs, but the same was true for 2% of hospitalized patients and only 0.02% of non-hospitalized COVID-19 patients. Even those non-hospitalized patients had a relatively higher rate of these clots compared to people who never caught covid-19 at all (0.004%). And while this risk may be small in general for mild COVID-19 cases, the sheer number of infections still means that a sizable number of people in this group can experience these and other serious health problems.

One important limitation to this and other studies measuring the odds of longer term health problems linked to COVID-19 is its timing. By late May 2021, Sweden had only just opened up vaccine availability to people in their mid-50s, and it would take another month before a majority of adult residents were vaccinated. Studies have consistently shown that vaccines reduce the risk of serious and long-term symptoms, even in people who contract a breakthrough infection. So the exact numbers in this study may be less applicable to today’s situation, where much of the world has either been vaccinated and/or survived infection. The study did find that the odds of these problems showing up in people did decline over time, perhaps in part due to vaccination.

Regardless, there are many people who continue to experience severe COVID-19 as well as lingering symptoms attributed to infection, while many parts of the world remain undervaccinated. And the researchers say that more may need to be done to monitor for and avoid these specific problems in patients, such as offering preventive treatments for clotting to high-risk patients.