Pathophysiology of SARS-CoV-2 Infection of Nasal Respiratory and Olfactory Epithelia and Its Clinical Impact

Smell, taste and chemesthesis disorders in patients with the SARS-CoV-2 during Omicron variant pandemic in China

Chemosensory disorders (including smell, taste and chemesthesis) are among the established symptoms of COVID-19 infection; however, new data indicate that the changes in chemosensory sensation caused by COVID-19 may differ among populations and COVID-19 variants. To date, few studies have focused on the influence of the SARS-CoV-2 Omicron variant on qualitative changes and quantitative reductions in chemosensory function in China. We conducted a cross-sectional study of patients with COVID-19 caused by the Omicron variant, to investigate the prevalence of chemosensory disorders and chemosensory function before and during infection, using an online questionnaire. A total of 1245 patients with COVID-19 completed the survey. The prevalence rates of smell, taste, and chemesthesis disorders were 69.2%, 67.7%, and 31.4%, respectively. Our data indicate that sex, age, smoking, and COVID-19-related symptoms, such as lack of appetite, dyspnea, and fatigue, may be associated with chemosensory disorders during COVID-19. Self-rating of chemosensory function revealed that patients experienced a general decline in smell, taste, and chemesthesis function. Further longitudinal research studies are needed to generate additional data based on objective assessment and investigate the factors influencing chemosensory function in COVID-19.

The immune mechanism of the nasal epithelium in COVID-19-related olfactory dysfunction

During the first waves of the coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, olfactory dysfunction (OD) was reported as a frequent clinical sign. The nasal epithelium is one of the front-line protections against viral infections, and the immune responses of the nasal mucosa may be associated with OD. Two mechanisms underlying OD occurrence in COVID-19 have been proposed: the infection of sustentacular cells and the inflammatory reaction of the nasal epithelium. The former triggers OD and the latter likely prolongs OD. These two alternative mechanisms may act in parallel; the infection of sustentacular cells is more important for OD occurrence because sustentacular cells are more likely to be the entry point of SARS-CoV-2 than olfactory neurons and more susceptible to early injury. Furthermore, sustentacular cells abundantly express transmembrane protease, serine 2 (TMPRSS2) and play a major role in the olfactory epithelium. OD occurrence in COVID-19 has revealed crucial roles of sustentacular cells. This review aims to elucidate how immune responses of the nasal epithelium contribute to COVID-19-related OD. Understanding the underlying immune mechanisms of the nasal epithelium in OD may aid in the development of improved medical treatments for COVID-19-related OD.

Prevalence of Olfactory Dysfunction with the Omicron Variant of SARS-CoV-2: A Systematic Review and Meta-Analysis

The omicron variant is thought to cause less olfactory dysfunction than previous variants of SARS-CoV-2, but the reported prevalence differs greatly between populations and studies. Our systematic review and meta-analysis provide information regarding regional differences in prevalence as well as an estimate of the global prevalence of olfactory dysfunction based on 62 studies reporting information on 626,035 patients infected with the omicron variant. Our estimate of the omicron-induced prevalence of olfactory dysfunction in populations of European ancestry is 11.7%, while it is significantly lower in all other populations, ranging between 1.9% and 4.9%. When ethnic differences and population sizes are considered, the global prevalence of omicron-induced olfactory dysfunction in adults is estimated to be 3.7%. Omicron's effect on olfaction is twofold to tenfold lower than that of the alpha or delta variants according to previous meta-analyses and our analysis of studies that directly compared the prevalence of olfactory dysfunction between omicron and previous variants. The profile of the prevalence differences between ethnicities mirrors the results of a recent genome-wide association study that connected a gene locus encoding an odorant-metabolizing enzyme, UDP glycosyltransferase, to the extent of COVID-19-related loss of smell. Our analysis is consistent with the hypothesis that this enzyme contributes to the observed population differences.