Blood clot risk remains high nearly a year after COVID-19

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In a study from the University of Bristol, scientists found people who got COVID-19 had a higher risk of dangerous blood clots close to a year later.

They examined the aftereffects of a SARS-CoV-2 infection during the period before vaccines became available.

As seen in previous studies, COVID-19 was linked to a sharply increased risk of blood clot-related issues – including heart attack and stroke – immediately after diagnosis compared to people who never had COVID-19.

But the new study found that risk remained higher for some problems up to 49 weeks later.

In the study, the team found at that point, the risk of deep vein thrombosis – clots that form in large veins – was nearly double in people who’d had COVID-19 compared to those who had not.

They used data collected anonymously from 48 million people – nearly every adult in England and Wales – in Great Britain’s National Health System from January 2020 until the day before COVID-19 vaccines were made available that December.

Researchers found that in the first week after a COVID-19 diagnosis, the risk of an arterial blood clot – the kind that could cause a heart attack or ischemic stroke by blocking blood flow to the heart or brain – was nearly 22 times higher than in someone without COVID-19.

That risk dropped sharply, to less than four times higher, in the second week.

In the first week after a COVID-19 diagnosis, the risk of such venous problems was 33 times higher. By the third and fourth weeks after diagnosis, the risk was still about eight times higher.

And between 27 and 49 weeks later, the risk was still 1.8 times higher than in somebody who had never had COVID-19.

Elevated risks persisted no matter whether someone was hospitalized for COVID-19, although risks were greater in people who were hospitalized. The study also showed that clot risks were higher in Black and Asian people.

Overall, however, clots were rare. The overall increase in the risk of developing an arterial clot in the 49 weeks after a COVID-19 diagnosis was 0.5%.

For a venous clot, the risk was 0.25%. After 1.4 million COVID-19 diagnoses, that corresponds to about 7,200 additional heart attacks or strokes, and 3,500 additional cases of pulmonary embolism, deep vein thrombosis, or other venous problems.

Although several variants of the coronavirus that causes COVID-19 have emerged since the time of the study, the team says the information remains relevant.

Given that risks can be elevated for close to a year, people should be screened and monitored for any evidence of risk factors that could be mitigated by preventive therapies, such as blood-thinning medications.

If you care about COVID, please read studies about the possible root cause of severe COVID-19, and COVID-19 vaccination is more effective than natural immunity.

For more information about COVID, please see recent studies that some blood types could cause severe COVID-19, and results showing very strong brain abnormalities post-COVID.

The study was conducted by Jonathan Sterne et al and published in the journal Circulation.

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