A lost sense of smell is one of the odder symptoms of COVID-19. How often does it really happen? A new study published in the Journal of Internal Medicine puts some numbers to it. The study is based on a review of 2,581 patients seen at 18 hospitals in Europe, according to a press release. Those with severe to critical cases reported the symptom infrequently: just 6.9% of the time. Ditto moderate cases, at 4.5%. But the lion's share of those with mild cases experienced it—85.9%. And it stuck around. The symptom lasted 21.6 days on average, though 24.1% of patients "did not subjectively recover olfaction 60 days after the onset of the dysfunction," per the study.
In addition to the self-reported data, the study considered objective clinical evaluations among a subset of 233 patients, which also found olfactory dysfunction was more prevalent in mild cases. The study authors hypothesize that increased prevalence is due to differences in immune responses. Their takeaway: "The high prevalence ... supports the need for primary care, ear, nose and throat (ENT) and neurology physicians to be able to counsel patients regarding the likelihood of recovery, and to identify those at risk of persistent OD." NBC News has a clear explainer of why experts think smell is affected to begin with: It relates to the cells that help direct neurons to the brain when we smell something; a receptor known as ACE2 covers those cells, and ACE2 is what COVID-19 goes after. And "there is no guarantee that those nerve connections will ever find their way back to their normal pathways." (Read more COVID-19 stories.)