What is the effect of the Omicron variant on olfaction?

By Dr. Liji Thomas, MDReviewed by Danielle Ellis, B.Sc.Sep 15 2022

The coronavirus disease 2019 (COVID-19) pandemic was declared early in 2020, following the emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December of the previous year. Soon after that, observant analysts began to note the frequent reporting of anosmia or hyposmia, abnormalities of the sense of smell, among COVID-19 patients, especially since this is not a common symptom with other lower respiratory infections.

A new study discusses the effect of the Omicron variant of the virus on the olfactory system in humans.

Introduction

The emergence of multiple variants of the viral pathogen that triggered the current pandemic has further prolonged its existence by allowing for immune escape via the modification of key epitopes on the viral spike protein and other important viral antigens. Increased transmissibility and/or enhanced virulence have also driven the rise of several such variants to prominence.

Beginning with the Alpha variant, which displaced the D614G mutation-containing strain that had, in turn, taken over from the ancestral or Wuhan variant of the virus, several more variants spread locally or globally. These included the Beta variant in and around South Africa, the Gamma variant in South America, the Delta variant that rapidly became dominant and caused fresh waves of hospitalization and death worldwide, and the Omicron variant.

Omicron has shown itself to be markedly more transmissible and infectious but less easily spreads to the lungs, accounting for the reduced rates of severe and fatal disease. These changes are probably traceable to the large number of mutations in the key spike protein of the virus. Of course, the underlying population immunity from earlier infections with this or other variants of the virus, and large-scale vaccination with one of the COVID-19 vaccines, are also contributors to this desirable outcome.

Once anosmia was declared a significant symptom of COVID-19, unlike rhinorrhea and other more typical symptoms of upper respiratory viruses that were conspicuously uncommon in this condition, it has been used for screening patients for possible COVID-19.

In contrast to earlier variants, however, Omicron has a different presentation. The current study, posted to the website of the International Forum of Allergy and Rhinology, reports on the olfactory symptoms of over 200 COVID-19-positive individuals, all of whom were infected by the Omicron variant.

The study period ran from December 21, 2021, to January 10, 2022.

What did the study show?

The findings of this study showed that of over 200 subjects infected with Omicron, of whom two-thirds were male, over 60% had a stuffy nose, and slightly fewer had a runny nose. Tiredness, fever, muscle aches, a feeling of being unwell, and coughing were present in over half the participants.

About one in seven had a history of COVID-19 before the current episode.

Interestingly, despite the high incidence of nose congestion, almost 70% reported an intact sense of smell. Less than a fifth had mild alterations, while less than one in seven reported complete loss of smell.

Since taste is a faculty powerfully dependent on smell, this was also inquired after. This showed that almost the same pattern was observable for the sense of taste in this group.

The incidence of hyposmia and anosmia within the Omicron cohort is thus lower than that reported in earlier studies on individuals infected with non-Omicron variants. Within this cohort, too, those who had an earlier episode of COVID-19, that is, reinfections, had a still smaller incidence of altered olfactory sensation compared to the rest. That is, less than a fifth had hyposmia, but only about one in 20 had anosmia.

What are the implications?

Our study findings support the decreased observation of isolated smell and taste loss with the omicron variant in comparison with previous variants of SARS-COV-2.”

Due to the lower affinity of Omicron for the lower airways, it causes mostly upper respiratory symptoms. Thus, patients with Omicron infection may present with nasal congestion, headache, tiredness, and sore throat as well as a cough, similar to the presentation of other endemic human coronaviruses like OC-43.

It is necessary to understand and emphasize this shift in presentation to ensure recognition of COVID-19 cases even without anosmia, as occurs in up to 80% of patients at present. Anosmia has been associated with long-term olfactory loss in a small proportion of cases.

It is gratifying to note that this change in clinical presentation will “lead to a significant reduction in the long-term morbidity associated with COVID-19 by decreasing the overall percentage of COVID-19 patients who will suffer from long-term severe olfactory loss.”