ARI SHAPIRO, HOST:

Loss of smell has become a hallmark of COVID-19. Up to 80% of infected people experience it. While most people get their sense of smell back as they recover, some do not. And, as Will Stone reports, this phenomenon has triggered new interest and research studies in the field of smell.

WILL STONE, BYLINE: There are some smells that college student Bo Anderson (ph) really misses - the fresh cut grass of the baseball field before a game, his mother's eggs and bacon mingling with pine on Christmas morning, even...

BO ANDERSON: As gross as my cat smells sometimes, you know, growing up with cats, I love them. So yeah, I miss that.

STONE: Anderson is a computer engineering major at Washington University in St. Louis. He got COVID-19 in the fall, and it was mild, mostly just headaches. A few days into quarantine, he woke up and couldn't smell anything.

ANDERSON: I went in the bathroom and smelled my deodorant, and there was nothing. You know, I thought in the next week I was going to get it back, but it's just never happened.

STONE: Which is why Anderson sits down at his kitchen table every day, opens a small jar of essential oils, brings it right up to his nose and sniffs...

(SOUNDBITE OF SNIFFING)

STONE: ...And sniffs...

(SOUNDBITE OF SNIFFING)

STONE: ...Until...

ANDERSON: For that one, I really couldn't get a whole lot from it. But here and there, I'll get a slight hint of oaky (ph) like tree bark.

STONE: That one was clove. Next, it's eucalyptus.

(SOUNDBITE OF SNIFFING)

ANDERSON: It makes my nose tingle.

(SOUNDBITE OF SNIFFING)

STONE: Then lemon.

(SOUNDBITE OF SNIFFING)

ANDERSON: Smells like sweaty socks mixed with like ice water with a lemon in it.

STONE: And finally, rose.

(SOUNDBITE OF SNIFFING)

ANDERSON: I can get a nice hint of it.

STONE: While Anderson sniffs, he also looks at pictures of what he's smelling. He does these smell retraining exercises every day and records the results. It's for a study on post-COVID patients who've lost their sense of smell. The clinical term for that is anosmia. Dr. Jay Piccirillo is leading this study. He's an ear, nose and throat specialist at Washington University School of Medicine.

JAY PICCIRILLO: Prior to COVID, doing research with anosmia, particularly viral-associated anosmia, was rather lonely.

STONE: Doctors have known for years that other viral infections can trigger loss of smell, but it was rare, so it was difficult to find enough patients to actually study. And key details were hard to pin down, like when the loss of smell started or what caused it. But all of this is changing with COVID-19.

PICCIRILLO: The magnitude of the problem, like complete loss of smell, the number of people affected and the number of people who have persistent problems, this is just unheard of.

STONE: Most people who lose their sense of smell from COVID-19 do recover within two months. But in a subset of patients - estimates are 5 to 10% - this problem persists. And because doing research was challenging before COVID, Piccirillo says there are few treatments and lots of questions even with smell retraining exercises.

PICCIRILLO: We don't really have a good answer about, you know, how effective is it, and how effective is olfactory training above and beyond just doing nothing?

STONE: Researchers do have a working theory for how the virus may steal your smell. Robert Datta is a neuroscientist at Harvard Medical School. His team has identified which cells in the nose the virus appears to target. They're called support cells. They surround another kind of cell, sensory neurons, which send information about smell to the brain.

ROBERT DATTA: The support cells get infected. Your sense of smell is disrupted. But then, when the support cells repair themselves, you regain your sense of smell.

STONE: This is what happens to most people when they recover from the infection. But in the group who do not get their smell back...

DATTA: When the sensory neurons that are responsible for smell aren't being supported, they go on to die.

STONE: And it's possible the nerve connecting the nose to the brain also atrophies. It's not clear why this would only happen in certain people and not others. But Datta says the good news is that, for some people at least, these connections can be rebuilt.

DATTA: There are patients that have been infected with other viruses that have gone on to recover their sense of smell after a year or even two years. So it's absolutely possible that these patients will go on to recover their sense of smell.

STONE: This is all still preliminary, but Datta says COVID just underscores our ignorance about how disease affects our sense of smell. Steven Munger directs the University of Florida Center for Smell and Taste.

STEVEN MUNGER: I do think it has been neglected, in part because this sort of lack of understanding of how much it impacts individuals' lives.

STONE: You can't smell smoke or gas. How do you tell if the food in your fridge is starting to spoil or if you smell? And, of course, smell is intimately connected to taste, to memories, to intimacy.

MUNGER: There's a very big emotional component to smell that is usually not as well understood by anybody unless it's actually happened to you.

STONE: And one unexpected legacy of the pandemic is that millions of people now have to cope with this loss.

For NPR News, I'm Will Stone. Transcript provided by NPR, Copyright NPR.