Loss of Sense of Smell May Be Best Sign of COVID-19, Studies Find

Our sense of smell is a complex and nuanced biological process. According to two large international studies conducted by Alexander Wieck Fjaeldstad, MD, a researcher and professor at Aarhus University in Denmark, losing your sense of smell is also one of the most reliable indicators of a COVID-19 infection.

The studies surveyed 4,039 COVID-19 patients from 41 countries and confirmed what scientists have observed since the beginning of the pandemic—losing your sense of smell is a reliable early indicator of COVID-19 infection.

Verywell spoke to Eric Holbrook, MD, the director of the division of rhinology at Massachusetts Eye and Ear and associate professor at Harvard Medical School, about how COVID-19 affects the olfactory nerves and why your sense of smell may not return right away after you've had the virus.

COVID-19 Smell Loss Is Universal

Holbrook tells Verywell that while other viruses can also cause smell loss, in COVID-19 patients, the loss of the sense in the case of COVID-19 is universal instead of targeting certain populations, like older people.

"It's really affecting the younger population," Holbrook says. "The number of teenagers and early adults that are suffering from smell loss after the virus is much higher than what I've seen in the past."

Smell loss associated with COVID-19 is also unique because it is related to the lack of ability to smell odors rather than being caused by a physical blockage such as excess mucus.

The Role of Your Smell Receptors

The COVID-19 virus is spread by droplets in the air. Your nose takes in the air as you breathe, making it a prime route for the virus to enter your body.

Although ACE2 receptors are not found on the actual olfactory neurons within the nose, they are present in the supporting cells (sustentacular cells). It's when those cells are attacked that the sense of smell is affected.

"It's not a direct attack on the neurons if you go by the ACE2 receptor theory," Holbrook says. "But that might be why you have certain people that have a short term loss because if it's just those supporting cells that are infected and damaged, and the olfactory neurons live, they just have to wait for those supporting cells to come back. That's a lot easier to recover from than direct damage." 

Sense of Taste

Many people with COVID-19 who were part of the recent studies also reported losing or having changes to their sense of taste. Holbrook says this makes sense because the ability to taste actually relies on multiple sensory inputs.

The sense of taste is often confused with the concept of flavor. Holbrook explains that in its simplest form, your ability to taste relies on information collected by the taste receptors on your tongue and the back of your mouth that detect sweet, salty, sour, bitter, or umami flavors.

In contrast, flavor combines information gathered by your tongue and olfactory nerves, as well as nerves that detect texture, such as the trigeminal nerve. Even visual information—what you see—can affect how flavor is perceived.

COVID-19 attacks the olfactory nerves, which is why many patients report having lost their sense of taste and smell. In rare cases, Holbrook says that some patients report that food has a different mouthfeel, indicating that the trigeminal nerve is also affected.

When Will Smell Return?

Holbrooks says that while olfactory nerves are some of the only sensory nerve cells in the body with regenerative ability, it's a slow process. When damage is extensive, recovery of full olfactory capability can take months—if not years.

Hyposmia—partial smell loss—seems to be a widespread effect of COVID-19, and it is probably even more widespread than data-driven studies like Fjaeldstad's can accurately reveal.

Some people with COVID-19 do not experience smell loss. Holbrook says that our sense of smell is not an all or nothing sensation—even patients who do not have a complete loss of smell may have diminished capabilities to detect odors, especially when tested using odor concentration tests rather than data based on perceived ability.

Most people who get the virus regain their sense of smell within two to three weeks, but COVID-19 "long-haulers" may deal with the loss for months.

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Helping Patients Recover

Scientists are working on how to help people who have persistent loss of smell after having COVID-19. Olfactory training involves giving patients a kit with four different scents and having them smell each one for ten seconds, twice a day. The treatment has shown promise for treating other cases of hyposmia or dysosmia.

One recent study by the Université Libre de Bruxelles in Belgium found that a combination of olfactory training and corticosteroids may be a promising technique for helping people with long-term smell loss.

A small sample of 27 patients was split into two different groups. Both groups took an initial smell assessment test five weeks after the onset of their smelling loss. One group practiced olfactory training. The second group did the same olfactory training plus a ten-day regimen of oral corticosteroids.

After ten weeks, the participants were retested. The group that took the oral steroids and completed the olfactory training had a significantly higher recovery rate than the group that only did the training. Corticosteroids are not a safe treatment option for everyone, but the study indicated that they might help some people with hyposmia.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.