Loss of smell (anosmia) is a common symptom of COVID-19, reported in more than 70% of patients with mild infection. The exact mechanism leading to loss of smell is unclear, although research suggests that viral invasion of the olfactory epithelium – sensory cells in the roof of the nasal cavity, might be a probable cause. Since sensory neurons responsible for olfaction does not have the receptor that COVID-19 exploits (ACE2) to enter human cells, they remain unaffected. Fortunately, this means patients likely regain their sense of smell as soon as the epithelium regenerates. But how long does that take?
One study by Lechien et al. investigated recovery of olfaction in 2581 patients diagnosed with COVID-19 in Europe. The sense of smell in patients was rated using either subjective evaluations (N = 1363) or objective evaluations using the Sniffin-Sticks test (N = 233). This study found that a third of the patients recovered olfaction after 14 days of losing it. After two months, the proportion of patients having recovered olfaction rose to 75% – 85 %.
Another study, led by Lucia Iannuzzi and Anna Eugenia Salzo performed subjective and objective evaluations of olfaction in 30 patients in Germany. Patients in the study were tested during hospitalization and about two months after symptoms onset. This study also found a substantial recovery of olfaction in patients two months after the onset of anosmia, although improvements started showing around a month after onset.
The proportion of people whose sense of smell is compromised is surprisingly high among COVID-19 patients with mild versions of the disease as opposed to the moderate and serious versions. Proportions were 85.9%, 4.5%, and 6.9% in mild, moderate, and severe-to-critical patients, respectively, in the first study cited (Lechien et al., 2021). Thus, losing one’s sense of smell is contingent on the severity of the disease. One can expect improvement in olfaction starting anywhere from 14 – 30 days after onset. A return to normalcy (full recovery), will likely take two months.
