Whereas fever, cough, and shortness of breath have characterized the disease caused by the novel COVID-19, the Centers for Disease Control and Prevention (CDC) updated its list of common symptoms in late April to include a new loss of smell or taste.
According to Justin Turner, M.D., Ph.D., associate professor of Otolaryngology-Head and Neck Surgery and medical director of Vanderbilt University Medical Center’s Smell and Taste Center, it’s not uncommon for patients with viral upper respiratory infections to experience a temporary—or sometimes permanent—loss of taste or smell, but these symptoms appear to be particularly prevalent in COVID-19.
Since COVID-19 is a new disease, little is known about the long-term outcomes of patients with these symptoms, but ongoing studies have provided insight into when these symptoms arise and who experiences them.
How common is smell and taste loss in COVID-19?
Up to 80% of people who test positive for COVID-19 have subjective complaints of smell or taste loss. That percentage rises when these patients are tested using objective methods that measure smell function. Most patients first notice problems with their sense of smell, but because smell is necessary to taste flavor, the symptoms are often connected.
What is known about the COVID-19 cases where these symptoms occur?
A recent study based on retrospective data showed that patients who had normal smell function in COVID-19 appeared to have a worse disease course and were more likely to be hospitalized and placed on a ventilator. This suggests patients who experience smell dysfunction may have a milder infection or disease.
The data we have so far also suggests that in a substantial percentage of the COVID-19-infected population, smell loss can be one of the first—or only—signs of disease. It may precede symptoms that are more commonly associated with COVID-19, such as cough and fever. It has even been proposed that smell and taste loss could be a screening tool since these symptoms appear so early.
How can a virus cause smell and taste loss?
One possibility is that people with upper respiratory infections often have congestion, drainage, and other nasal symptoms that can block odor’s ability to reach the smell nerve, which sits at the top of the nasal cavity. But we believe the primary cause, particularly for people with extended or permanent loss of smell function, is that the virus causes an inflammatory reaction inside the nose that can lead to a loss of the olfactory, or smell, neurons. In some cases, this is permanent, but in other cases, the neurons can regenerate. That’s likely what determines which patients recover.
In COVID-19, we believe smell loss is so prevalent because the receptors for COVID-19 that are expressed in human tissue are most commonly expressed in the nasal cavity and in the supporting cells of the olfactory tissue. These supporting cells surround the smell neurons and allow them to survive.
Should people with smell and taste loss in the absence of other symptoms be concerned about COVID-19?
While smell and taste loss can be caused by other conditions, it warrants a conversation with your physician to determine whether you should be tested for COVID-19. We know smell loss is one of the first—and sometimes only—symptoms in up to 25% of people diagnosed with COVID-19. It could be unrelated, but it’s important to seek care, especially if these symptoms are prolonged.
What questions about these COVID-19 symptoms still need answering?
We plan to watch the recovery rate for these patients. We encourage people who have prolonged smell and taste dysfunction to be evaluated to help us understand if and when these symptoms resolve.
There is also concern that COVID-19 and its ability to enter the olfactory tissue could be a conduit for infection in the brain. I think we’ll learn more about that as we follow these patients over time.