It has been known since the pandemic emerged in early 2020 that one of the tell-tale signs of being infected with coronavirus is a loss of the sense of smell. While most COVID patients recover their sense of smell within a few weeks, for some it persists for months or even years.

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Now, researchers at Duke University Medical Center in the US think they may have figured out why: the infection causes an ongoing immune assault on nerve cells within the nose.

“One of the first symptoms that has typically been associated with COVID-19 infection is loss of smell,” said co-researcher Bradley Goldstein, an associate professor in Duke’s Department of Neurobiology.

“Fortunately, many people who have an altered sense of smell during the acute phase of viral infection will recover smell within the next one to two weeks, but some do not.

“We need to better understand why this subset of people will go on to have persistent smell loss for months to years after being infected with SARS-CoV2.”

The researchers analysed samples of nasal tissue taken during the biopsies of 24 patients, nine of whom were suffering from a loss of smell associated with long COVID.

They found that T-cells, white blood cells that play a key role in the immune system, were triggering a widespread inflammatory response in the olfactory epithelium. This is a small area found at the top of the nasal cavity where nerve cells that process smells are located.

The inflammation was on-going despite the absence of detectable SARS-CoV-2 virus.

“The findings are striking. It’s almost resembling a sort of autoimmune-like process in the nose,” Goldstein said.

“We are hopeful that modulating the abnormal immune response or repair processes within the nose of these patients could help to at least partially restore a sense of smell.”

The finding also suggests that a similar mechanism could be responsible for other symptoms of long COVID such as brain fog, shortness of breath and fatigue, the researchers say.

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Authors

Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.

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