Why do COVID-19 patients lose their sense of smell and taste – when does it return?

In early March, people in many countries around the world began experiencing COVID-19 symptoms. Many reported fever, chills, and cough. However, many people in the West had also begun experiencing a symptom that was not often reported in China, where the outbreak began: loss of smell and an associated major loss of taste!

COVID-19 and loss of smell and taste

Medically called anosmia, loss of smell is not uncommon in COVID-19 patients, occurring in 34-98% of patients. Most experience this symptom to varying degrees for a few weeks. Moreover, loss of smell and taste may, for diagnosing COVID-19, be the most distinctive symptom. If you have this symptom, you therefore have stronger evidence for suspecting COVID-19 and getting tested than with other symptoms. One study found that COVID-19 patients were 27 times more likely to lose their sense of smell than people with other viral upper respiratory infections. Psychological problems such as COVID-19-related anxiety and depression, and difficulties socializing, are also reported more often by people with smell and taste loss. Knowing that this troublesome symptom will be temporary should reassure patients. 

Loss of smell and the associated loss of taste can last several weeks even after all other COVID-19 symptoms disappear. According to the American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS), among those who lose their sense of smell, 27% of COVID-19 patients experience "some improvement" within about 7 days, while most feel much better within 10 days.

Annals of Allergy, Asthma & İmmunology published a retrospective study in July 2020 involving 1013 patients, reporting that loss of smell was an independent factor indicating a milder course of COVID-19. Smell loss was reported more frequently in younger people, women, those with prior allergic rhinitis or rhinosinusitis, and those with a high BMI; people with anosmia were hospitalized less often.

Anosmia reduces the ability to smell chemical leaks or spoiled food. Meals are also one of our most important means of socializing. Being unable to participate fully can create a kind of social void.

How does smelling work?

The olfactory system, which allows humans and other animals to smell, is essentially a way of decoding chemical information. When a person detects a strong smell, molecules travel through the nose to the olfactory epithelium, a small piece of tissue at the back of the nasal cavity. These molecules bind to olfactory sensory neurons (olfactory nerves), and smell signals are then transmitted through bundles of nerves to the relevant parts of the brain. Smell molecules are thus identified as pleasant, burnt, floral, minty, or foul-smelling.

People may lose their sense of smell after a viral infection such as flu or a cold, or after traumatic brain injury. Some are born without any sense of smell; others lose it because of cancer treatments or diseases such as Parkinson's and Alzheimer's. The sense of smell may also diminish with age. Although smell disorders are less obvious than hearing or vision loss, data from the US National Institutes of Health (NIH) show that around 25 percent of Americans over 40 report some change in their sense of smell. This means that, in the United States alone, an estimated 13 million people have anosmia, complete smell loss, or hyposmia (partial loss).

COVID-19 anosmia

It is unclear whether COVID-19 anosmia differs from other instances of virus-induced smell loss, but people experiencing anosmia due to COVID-19 differ in several ways:

The most obvious difference is that they notice the loss immediately because it is usually not accompanied by the congestion that characterizes the early stages of virally induced anosmia. In other words, smell loss occurs without obvious nasal congestion.

This sudden, rapid smell loss is very dramatic, and these patients report being unable to smell anything.

Another notable difference is that many COVID-19 patients reporting smell loss regain this sense relatively quickly, within only a few weeks rather than months or years, unlike most people who experience anosmia from other viruses.

By what mechanism does COVID-19 affect the ability to smell?

Scientists have only just begun investigating why COVID-19 causes smell loss and whether it affects the olfactory system differently from other viral invaders.

Sandeep Robert Datta, a professor of neurobiology studying smell at Harvard University, says: “We know very little about how smell changes when we catch a cold.” What is your likelihood of experiencing smell loss? How long does it last? We do not actually know this basic thing."

The nose is one of the main places where the virus causing COVID-19, SARS-CoV-2, enters the body. Researchers have identified two types of nasal cells (goblet and ciliated cells) that are likely entry points for the virus.

To understand how COVID-19 disrupts the olfactory system, scientists first narrowed down the list of potentially infectious targets. To enter a cell, SARS-CoV-2 needs ACE2 and TMPRSS2, two proteins. Researchers therefore looked for cells in the olfactory system containing these two proteins.

They discovered that the nerve cells that transmit smell are not susceptible to coronavirus because they do not carry these two cell-surface proteins. Science Advances published an article reporting that ACE2 and TMPRSS22 are found not in neurons but in other olfactory-system cells, including cells supporting neurons and stem cells that regenerate neurons after damage. These supporting cells help physically support neurons, clear dead cells, and move odors through mucus in the system.

Understanding which parts of the olfactory system are affected is a major first step toward understanding how the virus disrupts our ability to perceive odors. However, knowing which cells are targeted appears to be only one piece of a much larger puzzle.

It is not yet known whether olfactory dysfunction results directly from the virus's damaging effects on neurons or from inflammation caused by the immune response to the virus.

Associate Professor Dr. Jane Parker, a flavor chemist at the University of Reading, and Dr. Simon Gane, a rhinologist at the University of London, published an article in The Conversation suggesting that smell may have been lost because tissue swelling in the airways compresses the olfactory epithelium and nerves that enable the nose to smell. They state that the sense of smell may return to its previous state as the effects of infection and inflammation and/or edema in this region diminish.

Even with some early clues, many questions remain unanswered. Why do some people recover within a few weeks while others do not? Does COVID-19 cause permanent smell loss in some people? And does the virus pass from the olfactory system to the brain?

Treatment for smell loss

There is currently no specific treatment for smell loss. However, there are some attempts.

One possibility is a kind of physical therapy for your nose called smell training. Some practitioners try a few small, gentle sniffs from bottles of essential oils such as eucalyptus, clove, or lemon every day. While sniffing, they try to focus on the smell even if they cannot detect it.

A body of evidence suggests that smell training may be effective, but there is no guarantee that it will work for everyone, and there is no research on whether it works for COVID-19 smell loss. Nevertheless, researchers believe that trying it does no harm and may be beneficial.

In conclusion Loss of smell and taste is a distinctive and important symptom of COVID-19, and many patients experiencing it have been observed to regain their senses within 1-3 weeks.

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