A new study has found that not only is a reduced sense of smell one of the most common symptoms of coronavirus disease 2019 (COVID-19), but it is significantly more common among patients with mild cases.

The study, published in the Journal of Internal Medicine, included 2581 patients from 18 European hospitals, including 2194 patients with mild COVID-19, 110 with moderate disease, and 277 with severe-to-critical disease. The study authors noted a higher proportion of women in the mild patient group compared with the other groups.

Patients with moderate or critical disease had higher prevalence of several comorbidities, including hypertension, diabetes, gastric disorders, and renal, respiratory, heart, liver, or neurological disorders. Olfactory dysfunction was more prevalent in those with mild disease, according to both the self-reported data and the objective clinical evaluations. Patients with severe and critical disease more frequently had cough, dyspnea, and fever.

Investigators found that the patient-reported prevalence of olfactory dysfunction was 85.9% in mild cases of COVID-19, 4.5% in moderate cases, and 6.9% in severe-to-critical cases. Furthermore, the average duration of this dysfunction reported by patients was 21.6 days, although nearly one-quarter of affected patients reported that they still had not recovered their sense of smell after 60 days.

Notably, objective clinical evaluations found different rates of olfactory dysfunction. Specifically, they identified dysfunction in 54.7% of mild cases and 36.6% of moderate-to-critical cases. At 60 days and 6 months, 15.3% and 4.7% of these patients, respectively, did not recover their sense of smell based on the objective evaluations.

The authors said their main hypothesis for the higher prevalence of olfactory dysfunction in mild COVID-19 cases was based on differences in the immune response to the infection. Patients with mild disease could have a better local immunological response through a higher production of immunoglobulin A, which could limit the virus spread. Due to the local inflammatory reaction and the olfactory cell expression of angiotensin-converting enzyme-2 and TMPRSS2, the authors said the patients with mild COVID-19 could have more severe impairment of the olfactory cells.

The authors also noted that younger patients could have a higher rate of anosmia compared with elderly patients, and similar results have been found in other studies. All of these findings require further research, but the authors concluded that olfactory dysfunction is a prevalent disorder in patients with COVID-19 and has a higher prevalence in patients with mild disease.

Lechien JR, Chiesa-Estomba CM, Beckers E, Mustin V, et al. Prevalence and 6-month recovery of olfactory dysfunction: a multicentre study of 1363 COVID-19 patients. Journal of Internal Medicine; January 5, 2021. https://onlinelibrary.wiley.com/doi/10.1111/joim.13209. Accessed January 7, 2020.