Covid-19: Study links disease to increased risk of clot, thrombosis and embolism

A person who has had Covid-19 has a higher risk of developing a serious blood clot within six months of having the illness.

This is one of the main conclusions of a recent study carried out in Sweden and published in the specialized journal British Medical Journal (BMJ).

The research also found that people with severe Covid, especially those who had to be hospitalized, and those infected during the first wave had a higher risk of clots.

The study does not claim that Covid-19 was the direct cause, but it does identify infection as a risk factor for the development of blood clots within a blood vessel.

To reach these conclusions, the researchers tracked the health status of more than one million people who tested positive for Covid between February 2020 and May 2021 in Sweden and compared them to four million people of the same age and sex.

According to the authors of the research, their findings highlight the importance of vaccination.

the evidence

The study suggests that after a Covid-19 infection, there is an increased risk of developing:

• deep vein thrombosis (DVT), which are blood clots in the leg, within three months.
• pulmonary embolism, which are blood clots in the lungs, within six months.
• internal bleeding, such as a stroke, within a two-month period.

Comparing the risks of blood clots after Covid-19 with the normal level of risk, the scientists say that:

• four in 10,000 Covid patients developed DVT, compared to 1 in 10,000 people who did not have Covid.
• about 17 out of 10,000 Covid patients had a blood clot in their lung, compared to less than 1 out of 10,000 who did not have Covid.

The study argues that the increased risk of blood clots was greater in the first wave of the pandemic, likely because treatments improved in the following months and older patients began to be vaccinated in the second wave.

This result was “expected”, according to Inmaculada Roldán Rabadán, a cardiologist at the Cardiovascular Thrombosis Group of the Spanish Society of Cardiology, in statements to the Science Media Center Spain portal.

“We had fewer tools to manage the disease [na época]”, she explains.

The risk of blood clot in the lung in seriously ill people with Covid was shown to be 290 times higher than normal and seven times higher than normal after mild Covid.

Mild Covid cases did not increase the risk of internal bleeding.

‘Good reason to get vaccinated’

Blood clots can also occur even after vaccination, but the risk is much lower, according to a study by the University of Oxford in August 2021.

“For unvaccinated people, this is a good reason to get vaccinated: the risk [das doenças] is much greater than the risk of vaccines,” says Anne-Marie Fors Connolly, a researcher at the Department of Clinical Microbiology at the University of Umea, Sweden, and lead author of the study.

Frederick K Ho, a professor of public health at the University of Glasgow who was not involved in the study, argues that while the risk of blood clots increases after vaccination, “the magnitude of the risk remains lower and persists for a shorter period of time than associated with to infection.

Covid and clots

The study does not prove that Covid is the cause of blood clots.

With this type of study “we can only determine whether there is an association between Covid-19 and blood clots or bleeding”, Fors Connolly told BBC News Mundo, the BBC’s Spanish service.

The expert adds that, to establish a causal relationship, other types of studies would be needed.

“The data are clear in showing that there is an association (between Covid-19 and clots), but what is not entirely clear is how this association works,” Jon Gibbins, director of the University’s Cardiovascular Research Institute, told BBC News Mundo. from Reading, who was not involved in the research.

“More efforts are needed to determine whether this is due to a long-standing inflammatory condition or some form of long-lasting immune dysfunction,” adds Gibbins.

Still, researchers believe the clots could be caused by the virus’s direct effect on the layer of cells lining blood vessels, an exaggerated inflammatory response to the virus, or the body forming blood clots at inappropriate times.

Frederick K Ho says this study “reminds us of the need to remain vigilant for complications associated with even mild Covid infections, including thromboembolism.”

For his part, Roldán Rabadán concludes that this research “is highly relevant for the management of the disease in the future”.

* With information from Philippa Roxby and Carlos Serrano.