Researchers now know why some people recover their loss of smell after COVID-19 and some do not. 

A new study, published Wednesday in the journal Science Translational Medicine, shows that for some people, their body’s immune response becomes dysregulated, even after the virus can no longer be detected by laboratory tests. Specifically, COVID-19 can cause a prolonged and damaging inflammatory assault on nerve cells in the nose that are responsible for the sense of smell.

The study was small, with samples from 24 people split into three groups: people with post-COVID prolonged loss of smell, people with a normal sense of smell after recovering from the virus, and people who never had COVID and who had a normal sense of smell. 

“The findings are striking,” researcher Bradley Goldstein, MD, PhD, an associate professor at Duke University School of Medicine in North Carolina, said in a news release. “It’s almost resembling a sort of autoimmune-like process in the nose.”

Persistent loss of smell after COVID-19 can last years. The good news is that even patients most affected by the virus appeared to maintain the cell types needed to repair the sense of smell, the study found.

Goldstein said the findings point scientists toward treatments that “could help to at least partially restore a sense of smell.” 

He said his lab at Duke is trying to help develop those treatments. 

While the researchers set out to study what caused the prolonged loss of smell after COVID-19, their findings may also shed light on other symptoms of long COVID, they said.