A new UK study led by a research team from the University of Oxford and the University of Leicester has suggested that blood clots in the brain or the lungs could potentially explain common symptoms of long COVID.

The post-hospitalisation COVID-19 study (PHosp-COVID), published in Nature Medicine, found that people admitted to hospital due to COVID-19 had higher levels of the fibrinogen protein and protein fragment D-dimer, which can cause excess clotting.

Excess blood clotting was found to be a possible cause for symptoms like brain fog and fatigue in long COVID.
Previous research has found that blood clotting can cause a lack of blood supply to the brain or cause fatigue, which both can cause cognitive problems.

People with long COVID can have a wide range of symptoms that last weeks, months, or even years after having a SARS-CoV-2 infection, and for some, it can result in disability or other milder infections.

Study author, Dr Max Taquet, from the University of Oxford, said: "Both fibrinogen and D-dimer are involved in blood clotting and so the results support the hypothesis that blood clots are a cause of post-COVID-19 cognitive problems.

"Fibrinogen may be directly acting on the brain and its blood vessels, whereas D-dimer often reflects blood clots in the lungs and the problems in the brain might be due to lack of oxygen."

After tracking cognitive problems at six and 12 months through tests and questionnaires, the researchers also found that those with higher levels of D-dimer complained of extreme tiredness, shortness of breath and difficulty holding down a job.

In the UK, around 1.9 million people still experience long COVID, 37% of whom have difficulty concentrating.

Study author, Professor Paul Harrison, from the University of Oxford, explained that the identification of predictors and other possible mechanisms was "a key step" in understanding post-COVID-19 brain fog. However, there could still be many other causes.
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