COVID: Blood clot risks still higher a year after infection, study says

The risk of developing a blood clot remained elevated almost a year after a COVID-19 infection, according to a new UK study that looked at the health records of 48 million unvaccinated adults, covering almost the entire adult English and Welsh populations, from the first year of the pandemic.

In a peer-reviewed paper published by the American Heart Association’s Circulation journal, researchers estimated that in 2020, COVID-19 resulted in more than 10,500 additional heart attacks, strokes, and other blood clot-related events in England and Wales. While excess risks were overall small and fell over time, researchers still found that the chances of developing a venous thromboembolism (VTE) following a COVID-19 diagnosis remained almost twice as high for up to 49 weeks following a positive test, compared to people who were not diagnosed with the viral infection.

VTEs are blood clots in the veins, which the US Centers for Disease Control and Prevention says is a serious and underdiagnosed, yet preventable condition that can cause disability and death. A clot in the lungs is an example of VTE. Arterial thrombosis is a clot that develops in an artery and is also potentially dangerous.

“We are reassured that the risk drops quite quickly – particularly for heart attacks and strokes – but the finding that it remains elevated for some time highlights the longer-term effects of COVID-19 that we are only beginning to understand,” said the study’s co-lead, Jonathan Sterne, a professor of medical statistics and epidemiology at the University of Bristol, in a statement. Stern is also the director of the NIHR Bristol Biomedical Research Center and director of Health Data Research UK South West.

Like previous studies on blood clots, the team led by the Universities of Bristol, Cambridge, and Edinburgh, and Swansea University, also found that risks of developing vascular disease climbed “markedly higher” in the first one to two weeks following confirmation of COVID- 19, a risk that did decline over time. Unlike (the risk of?) arterial thromboses, which declined quickly following initial infection, however, VTE risks remained more elevated. Researchers did find that the relative incidences of both arterial thrombosis and VTE remained higher for longer especially in patients who were hospitalized.

Overall, the team found that within the first week of testing positive for COVID-19, patients were 21 times more likely to have artery-blocking clots, which can lead to heart attacks and stroke. This risk dropped to 1.3 times more likely sometime after six months. For VTE, elevated risks went from 33 times more likely in the first week, to 1.8 times between 27 and 49 weeks.

While there was little association between COVID-19 and blood clot risks by age, researchers did find that Black and Asian people, and those with a history of blood clots, were at higher risk compared to patients who were white. Those with only mild or moderate cases of COVID-19 were also affected, although their excess risk was generally lower than those with severe infections.

“We have shown that even people who were not hospitalized faced a higher risk of blood clots in the first wave,” said Angela Wood, a professor of biostatistics at the University of Cambridge and a co-lead in the study.

“While the risk to individuals remains small, the effect on the public’s health could be substantial and strategies to prevent vascular events will be important as we continue through the pandemic.”

The research team used anonymized electronic health records across the entire English and Welsh population from January 1 to December 7, 2020 to analyze the data, which included looking at the severity of a patient’s COVID-19 infection, the demographics of the patients and their medical History. The data collected would have been prior to the mass vaccination campaign and before variants like Delta and Omicron became dominant.

“The large number of COVID-19 infections in England and Wales during 2020 and 2021 is likely to have caused a substantial additional burden of arterial thromboses and VTEs,” the paper’s authors wrote, recommending that preventative strategies like a health review with a primary care doctor and managing high-risk patients could help reduce incidences of dangerous blood clots. They noted that the opposite occurred during the pandemic – fewer patients saw doctors, which led to fewer routine health checks for those with chronic medical issues and fewer prescriptions given to patients for drugs that can help lower blood pressure and cholesterol.

Researchers noted some limitations to the study, including the fact that patients who died in nursing homes from a blood clot-related event may not have been recorded as such, due to a lack of diagnostic resources for example. Some people experiencing milder cases of blood clotting may have also avoided going to the doctor or hospital due to concerns over catching COVID-19. The data collected also did not include information on certain milder forms of clots. In addition, the authors noted that testing for COVID-19 was not widely available for mild or asymptomatic cases during the early days of the pandemic.

Looking ahead, researchers are studying data from after 2020 to better understand how vaccinations and other variants impact vascular health.