Loss of smell (anosmia) became a recognizable sign of early COVID-19 infection. This inability to detect odors was often accompanied by a decreased sense of smell (hyposmia) or an altered sense of taste (dysgeusia). Some patients also developed parosmia, where familiar smells become distorted and often unpleasant. Early in the pandemic, this sudden chemosensory loss was a signature symptom, even appearing in asymptomatic individuals. This article examines the biological reasons behind this symptom, how its prevalence has changed with newer variants, and the recommended approach for prolonged olfactory issues.
The Mechanism: Why COVID Attacks the Sense of Smell
The SARS-CoV-2 virus requires the Angiotensin-converting enzyme 2 (ACE2) receptor and the transmembrane protease serine 2 (TMPRSS2) enzyme to initiate infection. Research shows the virus does not typically infect the olfactory sensory neurons that detect odors. Instead, ACE2 and TMPRSS2 are expressed abundantly on sustentacular cells, the support cells within the olfactory epithelium.
These sustentacular cells provide metabolic and structural support to the olfactory neurons. When the virus attacks and damages these support cells, it disrupts the function of the olfactory epithelium. This damage leads to inflammation and temporary dysfunction of the neurons, causing the sudden loss of smell.
Since the olfactory neurons are not directly destroyed, recovery is possible. The inflammation prevents the olfactory system from correctly sending scent messages to the brain. This non-neuronal infection explains why smell loss often occurs without the nasal obstruction typical of a common cold.
Shifting Symptom Profiles: Early vs. Current Variants
The prevalence of anosmia has shifted dramatically since the beginning of the pandemic. During the waves dominated by the Alpha and Delta variants, the loss of smell or taste was one of the most common presenting symptoms, affecting a large percentage of those infected. This high rate established olfactory dysfunction as a major indicator of COVID-19 infection.
With the emergence of the Omicron variant and its subsequent sub-variants, this symptom became significantly less frequent. Studies estimate that Omicron causes olfactory dysfunction at a rate two to tenfold lower than the earlier Alpha or Delta variants. This change is partly due to Omicron’s altered cellular preference.
The newer variants are less efficient at using the TMPRSS2 enzyme for cell entry. This means they infect the sustentacular support cells in the olfactory epithelium less effectively, causing less damage to the olfactory system. Globally, the prevalence of olfactory dysfunction with Omicron is estimated to be around 3.7% in adults.
This prevalence varies substantially by population, with higher rates observed in people of European ancestry. This difference suggests that host genetic factors may influence susceptibility to smell loss. While sore throat and fatigue are now more common symptoms, the loss of smell is no longer a primary indicator of infection for most people.
Managing Olfactory Dysfunction
For individuals who continue to experience a reduced or altered sense of smell after the acute infection resolves, the most recommended intervention is olfactory training. This process is akin to physical therapy for the nose, aimed at stimulating the neural pathways and encouraging recovery.
Smell training involves repeatedly sniffing a set of four distinct scents twice daily for 10 to 20 seconds each time. While sniffing, a person should concentrate on the memory and imagination of the scent to fully engage the brain’s olfactory centers. The traditional scents used are:
- Rose (floral)
- Lemon or citrus (fruity)
- Eucalyptus (resinous)
- Clove or cinnamon (spicy)
Most people who experience smell loss regain their sense of smell within two to three weeks. Olfactory training should be maintained for a minimum of three months for best results, as the neural recovery process is slow. If the sense of smell has not returned or improved within four weeks, consult an ear, nose, and throat (ENT) specialist to rule out other causes.