The loss or alteration of taste and smell became a distinctive and widely reported symptom of COVID-19. Unlike the congestion-related smell loss associated with a common cold, this sudden sensory disruption often occurred even when the nasal passages were clear, pointing to a direct biological mechanism. This phenomenon affects millions globally, significantly impacting quality of life by diminishing the pleasure of eating and creating safety concerns. The duration of these symptoms varies widely, with some individuals experiencing a swift return to normal while others face persistent changes that last for many months.
The Spectrum of Taste and Smell Disturbances
The impact of COVID-19 on the chemical senses is not a single, uniform experience, but rather a spectrum of distinct conditions. The most recognized symptom is anosmia, which describes the complete loss of the sense of smell. A related condition, hyposmia, refers to a reduced ability to smell.
When the virus affects the sense of taste, the corresponding condition is ageusia, meaning a total loss of taste perception. Dysgeusia is a different type of taste disruption, where the perception is altered, often leading to a persistent metallic or chemical taste. For many, the sensory loss evolves from a quantitative loss to a qualitative distortion. Parosmia is the medical term for a distorted sense of smell, where familiar odors are perceived as unpleasant, often described as smelling like sewage, burnt toast, or rotting food.
How COVID-19 Affects Sensory Cells
The virus responsible for COVID-19, SARS-CoV-2, causes sensory loss not by directly attacking the sensory nerves, but by infecting the supporting cells around them. The virus uses the angiotensin-converting enzyme 2 (ACE2) receptor to gain entry into cells. While olfactory sensory neurons, the cells that detect odors, do not typically express this receptor, the adjacent sustentacular cells in the olfactory epithelium do. Sustentacular cells are support cells that provide metabolic and structural maintenance to the delicate sensory neurons.
When SARS-CoV-2 infects these support cells, it triggers an intense inflammatory response in the nasal cavity. This inflammation and the resulting damage to the sustentacular cells can cause the olfactory sensory neurons to retract their cilia. The initial, rapid loss of smell is thought to be a consequence of this inflammatory environment and the loss of support function, rather than the death of the neurons themselves.
For many people, the sense of smell returns quickly as the sustentacular cells regenerate and inflammation subsides. However, in cases where the damage is more extensive, the sensory neurons can be more severely affected. This requires a longer process of nerve regeneration and re-wiring, which can lead to longer-term issues like parosmia.
Typical Recovery Timelines and Long-Term Outlook
The prognosis for the return of smell and taste after a COVID-19 infection is generally positive. Many patients report that their sensory function begins to resolve within one to two weeks of the acute infection. Within the first two months, a large majority of affected individuals, estimated to be around 88%, experience a substantial recovery of their sense of smell.
However, for a significant minority, the symptoms persist for much longer, sometimes for six months or more, a condition often associated with long-COVID. Even in these prolonged cases, recovery often continues for up to a year or longer, suggesting the ongoing process of nerve repair. An estimated 5% of adults who had COVID-19 may experience long-lasting changes to their smell or taste. If sensory loss persists or significantly impacts daily life, a consultation with an ear, nose, and throat (ENT) specialist may be beneficial.
Practical Strategies for Managing Sensory Loss
For individuals experiencing persistent sensory changes, a structured approach called olfactory retraining has shown promising results. This exercise involves smelling a small set of strong, familiar odors twice a day for a period of several weeks or months. The practice focuses not just on smelling the item, but also on consciously trying to recall and visualize the scent in the brain, encouraging the damaged neural pathways to reconnect. This process leverages the nervous system’s ability to heal and rewire itself, a concept known as neuroplasticity.
The standard scents used for retraining often include a rotation of four primary categories:
- A floral scent (like rose).
- A fruity scent (like lemon or citrus).
- A resinous scent (like eucalyptus).
- A spicy scent (like clove).
Beyond retraining, managing the condition involves practical adjustments to ensure safety and quality of life. The inability to detect odors creates a safety hazard, necessitating the use of working smoke alarms and careful attention to food expiration dates. When eating, focusing on other sensory elements, such as the texture, temperature, and visual appeal of food, can help make meals more enjoyable when taste is compromised. Adding strong flavors like chili or spices can also provide a distinct sensory input that bypasses the olfactory system.