(It's well known that having COVID-19 can affect your sense of smell, but in some cases, that olfactory function doesn't properly return. Now new research explains why.

The SARS-CoV-2 infection prompts an ongoing immune system attack on the nerve cells in the nose, the new study states, and there's then a decline in the number of those nerve cells, leaving people unable to sniff and smell as they usually would.

As well as answering a question that baffled experts, the research could also help our understanding of long COVID and why some people cannot fully recover from COVID-19.

"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," says neurobiologist Bradley Goldstein from Duke University in North Carolina.

"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-CoV-2."

The team studied nose tissue samples – olfactory epithelium – taken from 24 people, including nine experiencing a long-term loss of smell after having COVID-19. This tissue holds the neurons responsible for detecting odors.

After a detailed analysis, the researchers observed the widespread presence of T-cells, a type of white blood cell that helps the body fight off infection. These T-cells were driving an inflammatory response within the nose.

However, as with many other biological responses, the T-cells apparently do more harm than good and damage the olfactory epithelium tissue. The inflammation process was still evident even in tissue where SARS-CoV-2 wasn't detected.

"The findings are striking," says Goldstein. "It's almost resembling a sort of autoimmune-like process in the nose."

While the number of olfactory sensory neurons was lower in the study participants who had lost their sense of smell, the researchers report that some neurons seem capable of repairing themselves even after the T-cell bombardment – an encouraging sign.

The researchers suggest that similar inflammatory biological mechanisms could be behind the other symptoms of long COVID, including excessive fatigue, shortness of breath, and a 'brain fog' that makes it difficult to concentrate.

Next, the team wants to look in more detail at which particular tissue areas get damaged, and which types of cells are involved. That will, in turn, lead the way to develop possible treatments for those experiencing a long-term loss of smell.

"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," says Goldstein.

The research has been published in Science Translational Medicine.