The loss of smell, known as anosmia, and the reduction in the sense of taste, or hypogeusia, emerged as highly common and defining symptoms of COVID-19 infection. Unlike the smell loss seen with a common cold, which is typically caused by congestion, the anosmia associated with the SARS-CoV-2 virus is often sudden and profound. While the majority of individuals who experience this sensory loss recover quickly, a significant number have faced persistent dysfunction, turning this symptom into a major public health concern. This lingering effect has prompted extensive research into the biological mechanisms of the damage and the most effective ways to encourage recovery.
How the COVID-19 Virus Affects Olfactory Function
The SARS-CoV-2 virus does not usually attack the olfactory sensory neurons themselves, the nerve cells responsible for detecting odor signals. Instead, the virus targets the supporting cells within the olfactory epithelium, the tissue lining the nasal cavity. These are the sustentacular cells, which express the ACE2 receptor protein the virus uses for entry.
Sustentacular cells provide metabolic and structural support to the sensory neurons. When the virus infects these cells, it triggers an inflammatory response that disrupts the local environment. This inflammation temporarily incapacitates the neighboring olfactory neurons, preventing them from detecting odors. The disruption is often temporary because sustentacular cells have a faster regeneration rate than nerve cells.
Typical Recovery Timelines and Statistics
The question of recovery time has a wide range, but statistics provide a framework for the typical trajectory. For a large percentage of people, the loss of smell is short-lived, with studies indicating that between 60% and 74% of patients report a return of their sense of smell within the first 30 days after infection. This initial rapid recovery often corresponds to the resolution of the acute inflammation in the nasal lining.
Recovery typically continues at a strong pace over the following months, moving into the subacute phase. Approximately 78% of people self-report a complete recovery of their olfactory function within two months. This period of improvement extends further, with up to 95% of individuals reporting a full return of smell within six months of the initial infection.
For the remaining population, the smell loss is considered chronic, persisting beyond the six-month mark. Even in these long-term cases, the outlook remains positive for continued improvement, as the olfactory system has the capacity for slow regeneration. Recovery has been observed to continue for up to two years post-infection, though the process becomes much more gradual and can involve the slow re-establishment of neural connections.
Smell Training and Active Recovery Techniques
Active intervention is recommended to promote the regeneration and rewiring of the olfactory system. The most accessible method is olfactory training, often referred to as smell training. This involves a structured, repetitive process designed to stimulate and repair neural pathways.
The standard protocol requires repeated sniffing of four distinct categories of odors twice a day for a period of several months. The scientific rationale behind this practice is neuroplasticity, the brain’s ability to reorganize itself by forming new neural connections. These four categories are typically:
- Floral (like rose)
- Fruity (like lemon)
- Resinous (like eucalyptus)
- Spicy (like clove or cinnamon)
Patients are instructed to sniff each scent for about 20 seconds, concentrating intently on the odor and recalling associated memories to strengthen the brain-scent link. While smell training is the primary recommendation, some patients may also be prescribed therapeutic interventions, such as intranasal steroid rinses, to help manage residual inflammation. Smell training remains the most established and accessible technique to encourage recovery.
Navigating Parosmia and Phantosmia
As the olfactory nerves begin to regenerate, some individuals encounter qualitative distortions of smell that signal the system is actively rewiring. Parosmia is a condition where smells are perceived as altered or distorted, often turning pleasant or neutral odors into unpleasant ones, such as coffee smelling like burnt rubber or sewage. Phantosmia, a less common symptom, involves smelling odors that are not actually present in the environment, akin to an olfactory hallucination.
These distortions are generally viewed as a stage of recovery, indicating that the new neural connections are forming imperfectly before fully maturing. Parosmia and phantosmia can be highly disruptive to daily life, particularly impacting appetite and nutrition. Studies show that the prevalence of these qualitative changes increases several months after the initial infection, peaking as the system attempts to normalize.
Management often involves lifestyle modifications to minimize the impact of distorted smells. Since heat carries odor molecules more effectively, eating simple or bland foods at room temperature or even cold can help reduce the intensity of parosmic triggers. Continuing with modified smell training, even with distorted perception, is still encouraged to guide the nervous system toward correct function, as these symptoms typically diminish over time as recovery progresses.