An impaired sense of smell can persist for years following COVID, even when patients recover from their initial illness, new research in JAMA Network Open indicates.

The study enrolled more than 3000 people, most of whom had COVID, but some did not. Some participants suspected they had smell loss; others were less concerned. But many in either group had challenges with olfaction, which can be a life-threatening condition due to the inability to detect things like gas leaks or rotting food.

“When people tell you they have smell loss, they’re right,” said Leora Horwitz, MD, MHS, professor of population health and medicine at the NYU Grossman School of Medicine in New York City and co-author of the study.

The observational cohort study is part of the Researching COVID to Enhance Recovery project, funded by the National Institutes of Health to determine the long-term health effects and potential treatments after contracting SARS-CoV-2.

Horwitz and her colleagues enrolled 1393 adults with a history of COVID-19 who reported trouble with smell and 1563 who did not experience that complication of infection. Nearly 80% had olfactory difficulty shown through smell tests, and 23% of cases were severe. Most of these people (742) also reported cognitive challenges.

“The lack of smell goes hand-in-hand with cognitive deficits,” said Fernando Carnavali, MD, internal medicine physician and site director for the Center for Post-COVID Care at the Icahn School of Medicine at Mount Sinai in New York City.

Of the participants who did not report difficulty with smell, 1031 (66%) had some smell loss per the test. In this group, 128 (8.2%) of the impairments were severe despite the failure to report the problems.

The study also included 569 people who had never tested positive for COVID, of whom fewer than 40% had normal smell on testing.

On average, the smell test occurred 2 years after a SARS-CoV-2 infection. Horwitz said that the test was a snapshot in time and does not mean everyone with COVID will have impaired smell for 2 years. How long COVID-related smell impairments last; Horwitz said this is an area for further research.

“We think it’s sensible to do the test” as part of standard care after experiencing COVID, she added.

The most common smell test, the University of Pennsylvania Smell Identification Test, is a scratch-and-sniff assessment containing 40 different odors that patients smell and identify by multiple choice. Women who correctly identify 35 or more odors have a normal sense of smell; for men, correctly identifying 34 or more odors indicates normal smelling abilities.

Not Just COVID

Carnavali said he is still treating new patients for COVID. This study helps bring smell assessment in these patients back to the forefront, Carnavali said, particularly if they have some kind of cognitive challenge.

Experts caution that COVID is not the only cause of smell loss. The condition is sometimes idiopathic, with nothing to do with COVID, said Nyssa Farrell, MD, otolaryngologist at Washington University School of Medicine in St. Louis.

“Smell dysfunction is very common, and people don’t recognize it. Certainly, COVID is causing smell loss that’s unrecognized,” but so do many other things, Farrell said.

Trouble with smell can also come from tumors, traumas, or congenital conditions, among other causes, Farrell noted. And smell loss has long been associated with neurologic disorders such as Alzheimer’s and Parkinson’s disease.

In theory, Farrell said patients with COVID should receive a universal smell screening. But doing so is not trivial. The 40-item test could take a long time to administer, sometimes up to an hour in Farrell’s clinic. And it may not be covered by insurance, Farrell added.

Olfactory training has been shown to be modestly effective for patients with long COVID, Farrell said, although the degree of benefit is a matter of debate.

Horwitz, Carnavali, and Farrell reported having no relevant financial conflicts of interest.

Marcus A. Banks, MA, is a journalist based near New York City who covers health news with a focus on new cancer research. His work appears in Medscape Medical News, Cancer Today, The Scientist, Gastroenterology & Endoscopy News, Slate, TCTMD, and Spectrum.