Several researchers have found a common link between people who have undergone chemotherapy for any life-threatening disease and previously healthy people who have had a relatively mild COVID infection and are now facing brain fog.

A team from Stanford University was fascinated to find similar changes among patients in both groups, in specialised brain cells that serve as the organ's surveillance and defence system, the Washington Post reported. A malfunction in these cells known as microglia results in fuzzy thinking among cancer patients undergoing treatment. The same cells may also be impeded in Alzheimer's disease.

There's a growing body of research that suggests similarities in the mechanisms of post-COVID cognitive changes and other long-studied brain conditions, including 'chemo brain', Alzheimer's and other post-viral syndromes following infections with influenza, Epstein-Barr, HIV or Ebola, the report said.

"There is humongous overlap" between long COVID and these other conditions, Avindra Nath, intramural clinical director of the neurological disorders and stroke unit of the National Institutes of Health, was quoted as saying.

According to Stanford neuroscientist Michelle Monje, knowing the similarities gives the field of research into long COVID "a real foundation". She's optimistic that some of the symptoms people are experiencing post-COVID are reversible, and there's already talk about testing drugs in clinical trials to treat "chemo brain" for those suffering from severe COVID brain fog.

Another team of researchers from Harvard and Johns Hopkins University School of Medicine have highlighted how both in COVID -19 and chronic fatigue syndrome, too many oxygen molecules pile up in a cell - possibly resulting in inflammation that leads to cognitive issues.

And an examination of brain autopsy tissue by a Columbia University professor from 10 patients who died of COVID -19 turned up a molecular change bearing the distinct signature of Alzheimer's, the report said.

Researchers, however, are still in the early stages of defining COVID brain fog, the maddening feeling of slow thinking and confusion described post-infection by some people.