Anosmia (complete loss of smell) and ageusia (loss of taste) were common symptoms of infection with SARS-CoV-2, the virus that causes COVID-19. The inability to detect aromas or flavors significantly affects quality of life, reducing the enjoyment of food and creating safety risks, such as failing to detect gas leaks or spoiled food. While the prevalence of these symptoms has decreased with newer viral variants, millions have experienced this sensory disruption. Understanding the recovery process provides clearer expectations for those currently affected.
The Typical Recovery Timeline
For the majority of individuals who experience chemosensory loss, recovery begins quickly. Studies show that approximately 90% of those affected notice substantial improvement in their sense of smell within the first four weeks following the infection. In cases of rapid recovery, the median time until the return of smell is estimated to be around 11.5 days after the onset of symptoms.
The complete restoration of these senses can be a slower, more gradual process. Most patients who experienced persistent loss of smell or taste fully regain these senses within two years of the initial infection. Taste tends to recover faster than smell, suggesting different underlying mechanisms.
While the prognosis for recovery is high, a minority face a prolonged course. Data suggests about 24% of people who lost their sense of smell only experienced a partial recovery, and a smaller fraction reported no recovery. A long-term deficit can persist for weeks, months, or even over a year for some.
Understanding the Biological Cause of Sensory Loss
The loss of smell in COVID-19 is not caused by nasal congestion blocking airflow, which is the mechanism for the common cold. Instead, the SARS-CoV-2 virus directly targets and damages specific cells within the olfactory epithelium, the tissue lining the nasal cavity responsible for detecting odors. The virus gains entry into these cells using the angiotensin-converting enzyme 2 (ACE2) receptor, which is abundantly expressed on supporting cells, known as sustentacular cells.
The virus infects these sustentacular cells, which provide metabolic and structural support to the olfactory sensory neurons that transmit smell signals to the brain. The resulting inflammation and damage to these support cells cause the sensory neurons to become dysfunctional. Because the sensory neurons themselves are often spared from direct infection, the olfactory system has a high capacity for healing. Recovery depends on the regeneration of new, healthy sustentacular cells from stem cells, a process that takes time and explains the gradual return of smell.
When Recovery is Slow or Incomplete
When full recovery is delayed, individuals may experience qualitative changes to their sensory perception, indicating that the neural pathway is attempting to repair itself. One such distortion is parosmia, a condition where odors are perceived as unpleasant or distorted, often described as smelling rotten, chemical, or burnt. Common triggers like coffee, onions, or meat may suddenly smell disgusting, which can severely impact appetite and nutrition.
Another related condition is phantosmia, which involves smelling odors that are not actually present, such as phantom smoke or chemical fumes. Parosmia and phantosmia often appear two to three months after the initial infection, signaling that regenerating olfactory neurons are miswiring as they reconnect to the brain. While these distortions are distressing, parosmia is considered a sign of recovery, meaning the smell system is actively working toward normal function. If sensory loss persists for many months, or if new symptoms arise, seek a medical consultation to rule out other possible causes.
Strategies to Encourage Sensory Recovery
For those experiencing prolonged sensory loss, smell training, or olfactory retraining, is widely recommended. This technique is based on the neuroplasticity of the olfactory system, the ability of the nerves involved in smell to heal and regrow. The process involves the systematic, deliberate sniffing of a small set of scents daily to stimulate the healing olfactory nerves and help the brain relearn how to interpret the signals.
The standard training protocol involves using four distinct scent categories. Essential oils are commonly used for this purpose:
- Floral (e.g., rose)
- Fruity (e.g., lemon)
- Spicy (e.g., clove)
- Resinous (e.g., eucalyptus)
The process requires sniffing each scent for about 15 to 20 seconds, focusing on recalling the memory of the smell, and repeating the cycle twice a day. Consistency is important, and experts suggest continuing the training for at least three months before switching to a new set of four scents to maintain stimulation.