Blood clots in the brain or lungs might explain several typical symptoms of "long Covid," including brain fog and fatigue, a UK study has found.

Researchers believed two blood proteins studied in 1,837 persons referred to hospital due to Covid-19 indicate clots as one cause, reports BBC.

Also read: WHO-backed policy brief calls for action to address ‘long COVID’

It is estimated that 16 percent of such individuals will have difficulty thinking, concentrating, or remembering for at least six months, it said.

Long Covid may also develop as a result of less severe illnesses.

However, the study team from the universities of Oxford and Leicester emphasised:

— Their findings are solely applicable to hospitalised patients.

— They are "the first piece of the jigsaw" but further study is required before they can suggest or test any viable therapies.

—They only followed cognitive deficits at six and 12 months, using tests and questionnaires that may "lack sensitivity."

Identifying predictors and potential processes was "a key step" in understanding post-Covid brain fog, according to research author Prof Paul Harrison of the University of Oxford, said the report.

Also read: Clinic helps long-haul patients in London s COVID triangle

However, there might be other causes of long Covid, it added.

"It's a combination of someone's health before, the acute event itself and what happens afterwards that lead on to physical and mental health consequences," said Chris Brightling, a professor of Respiratory Medicine at the University of Leicester.

Dr Simon Retford, a university lecturer from Lancashire, spent two weeks in a coma after contracting Covid-19 in October 2020, with his family warned to anticipate the worst.

He is now 60-70 percent of his former self, although he still struggles with concentration, short-term memory loss, and losing his line of thought.

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"I took on a course-leader role last May and I was like a really slow computer which struggled to get going," he said.

The Post-hospitalisation Covid-19 study (PHosp-Covid) published in Nature Medicine blames brain fog on elevated levels of the protein fibrinogen and the protein fragment D-dimer, the report also 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 cognitive problems,” said Study author Dr Max Taquet, from Oxford.

"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," Taquet added.

Those with high D-dimer levels also complained of excessive fatigue and shortness of breath, and found it difficult to keep a job. A similar pattern was discovered in a research conducted in the United States, the report concluded.