The loss of smell, medically termed anosmia, became a defining symptom of COVID-19 for millions of people worldwide. This sudden inability to detect odors, often coupled with a loss of taste, was recognized early as a unique indicator of SARS-CoV-2 infection. Anosmia’s prominence set the disease apart from other common respiratory illnesses and highlighted the virus’s particular interaction with the human nervous system. Understanding the mechanism behind this symptom provides insight into the virus’s biological targets and helps inform recovery strategies.
Anosmia and COVID-19: What We Know
Smell loss is common, affecting 20% to 50% of infected individuals. It often appears early in the disease course, sometimes serving as the only presenting sign of infection.
A key distinction of COVID-19-related anosmia is that it typically occurs without the nasal congestion or runny nose associated with a common cold or flu. In other upper respiratory infections, smell loss is usually conductive, meaning the nose is physically blocked, preventing odor molecules from reaching the sensory area. With COVID-19, the nasal passages often remain clear, pointing toward a direct effect on the neural structures responsible for smell processing.
The Biological Mechanism Behind Smell Loss
The primary reason for smell loss in COVID-19 lies in the virus’s targeting of specific cells within the olfactory epithelium, the patch of tissue at the top of the nasal cavity. The SARS-CoV-2 virus must first attach to a host cell using the Angiotensin-converting enzyme 2 (ACE2) receptor and the TMPRSS2 protein. These two molecules act as the necessary entry points for the virus.
Scientific analysis revealed that the olfactory sensory neurons, which transmit smell information to the brain, do not express high levels of the ACE2 receptor. Instead, the virus preferentially targets the surrounding sustentacular cells. These supporting cells provide essential metabolic and structural support to the neurons.
When the virus infects these supporting cells, it triggers significant inflammation and damage within the olfactory epithelium. This disruption indirectly prevents the sensory neurons from functioning correctly, leading to the sudden loss of smell. This indirect attack explains why anosmia can be severe yet often temporary, as the sensory neurons themselves are spared from direct viral destruction.
The Recovery Timeline and Prognosis
For the majority of individuals, smell function returns relatively quickly, often within a few weeks of the initial infection. Research suggests that approximately 90% of patients see significant improvement within one to three months. This rapid resolution is consistent with the indirect mechanism of injury, where the supporting cells can regenerate and the inflammation subsides.
For a minority, impairment can persist for months, sometimes extending beyond a year. As the olfactory system attempts to repair itself, some individuals experience two related phenomena: parosmia and phantosmia.
Parosmia is a distortion of smells, causing previously pleasant odors (like coffee) to smell unpleasant or foul. Phantosmia involves phantom smells, where a person detects odors that are not actually present. These distortions are signs of the olfactory system misfiring as sensory neurons regrow and reconnect with the brain. The presence of parosmia can be viewed as a sign that the recovery process is underway.
Managing Smell Loss: Olfactory Training
Olfactory training, sometimes called smell therapy, is the most recommended intervention for prolonged or altered smell function. This practice is based on neuroplasticity, the brain’s ability to reorganize itself by forming new neural connections. The goal is to stimulate the olfactory system and encourage the repair of damaged pathways.
Olfactory training involves systematically sniffing a set of four distinct odors twice a day for several months. The scents traditionally used are essential oils representing four categories:
- Flowery (rose)
- Fruity (lemon)
- Spicy (clove)
- Resinous (eucalyptus)
Repeatedly exposing the olfactory system to these strong scents helps the recovering sensory neurons and the brain relearn how to process odor signals correctly. Consistent practice over 12 to 24 weeks can lead to measurable improvements.