Anosmia recovery rates in patients affected by COVID-19 is typically several weeks. However, other viral infections typically cause anosmia for months. This difference might be due to the lack of olfactory inflammation in some COVID-19 cases that is usually observed with other virus cases (including other coronaviruses).

Boston University

Anosmia and ageusia have been widely reported in COVID-19 patients in Italy, especially in younger patients. Out of 417 patients with mild to moderate COVID-19 in Milan, Italy, 85.6% reported olfactory dysfunction, and from 202 patients with mild COVID-19 symptoms in another hospital in Italy, 64.4% had impaired sense of smell or taste.
In Wuhan, China, among 214 patients hospitalized for COVID-19 symptoms, only 5.1% and 5.6% had impaired smell or taste, with impairment more prevalent among patients with mild rather than severe COVID-19 (6-7% versus 3.0%).


Reported Cases of Anosmia and Ageusia in COVID-19 Patients 

Brann, D., Tsukahara, T., Weinreb, C., Lipovsek, M., Van den Berge, K., Gong, B., ... & Robert Datta. (2020). Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID- 19-associated anosmia. https://advances.sciencemag.org/content/early/2020/07/28/sciadv.abc5801

Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID- 19-associated anosmia

ACE2 polymorphism: studies suggest that East Asian populations possess different allele frequencies of ACE2 than European populations, and that some ACE2 variants may have reduced ability to bind SARS-Cov1. Depending on which ACE2 type patients with COVID-19 have in different tissues, they may experience vastly different symptoms and outcomes in recovery.


Hypothesis for Significant Difference in Reported Anosmia or Ageusia between East Asia and European Communities

There has been speculation about the potential causes of anosmia and ageusia in COVID-19 patients. Two key MOAs have been proposed: a neuronal infection-based hypothesis and a non-neuronal infection-based hypothesis.

Potential Causes of Anosmia and Ageusia in COVID-19 Patients

Comparison of COVID-19-Related Anosmia and Other Viral-Related Anosmia 

- Anosmia is a result of congestion or the presence of edema in the nasal area
- When volunteers were vaccinated with a strain of the coronavirus, they experience nasal obstruction
- Due to infection, inflammation was presented in olfactory tissue. This then resulted in edema and rhinorrhea
- Further study on the nasal tissue found that there was a diminished cilia count and less olfactory neurons present (which could be an indicator of the inability to smell)
- Unlike the use of ACE2 in COVID-19 for the virus to bind to receptors on cells, the inoculation strain (HCoV-229E) uses aminopeptidase N
- Those with COVID-19 showed that the presence of increased amounts of ACE2 resulted in damaged cilia and goblet cells in the nasal tissue 
- In a survey done on patients with anosmia in South Korea, it was found that 30% of them had anosmia
- Out of 214 patients in Wuhan, 11 of them had anosmia
- An inoculation of a different strain of the coronavirus in mice through the nasal cavity showed that the virus could be detected in the olfactory bulb after 3 days
- Cytokines were present in the olfactory bulb
- COVID-19 could eventually spread to the blood-brain barrier would disrupt homeostasis in the brain, negatively impacting the olfactory area


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