A follow-up on quantitative and qualitative olfactory dysfunction and other symptoms in patients recovering from COVID-19 smell loss

Abstract

Background: Sudden smell loss is a specific early symptom of COVID-19, which, prior to the emergence of Omicron, had estimated prevalence of ~40% to 75%. Chemosensory impairments affect physical and mental health, and dietary behavior. Thus, it is critical to understand the rate and time course of smell recovery. The aim of this cohort study was to characterize smell function and recovery up to 11 months post COVID-19 infection.

Methods: This longitudinal survey of individuals suffering COVID-19-related smell loss assessed disease symptoms and gustatory and olfactory function. Participants (n=12,313) who completed an initial survey (S1) about respiratory symptoms, chemosensory function and COVID-19 diagnosis between April and September 2020, were invited to complete a follow-up survey (S2). Between September 2020 and February 2021, 27.5% participants responded (n=3,386), with 1,468 being diagnosed with COVID-19 and suffering co-occurring smell and taste loss at the beginning of their illness.

Results: At follow-up (median time since COVID-19 onset ~200 days), ~60% of women and ~48% of men reported less than 80% of their pre-illness smell ability. Taste typically recovered faster than smell, and taste loss rarely persisted if smell recovered. Prevalence of parosmia and phantosmia was ~10% of participants in S1 and increased substantially in S2: ~47% for parosmia and ~25% for phantosmia. Persistent smell impairment was associated with more symptoms overall, suggesting it may be a key marker of long-COVID illness. The ability to smell during COVID-19 was rated slightly lower by those who did not eventually recover their pre-illness ability to smell at S2.

Conclusions: While smell ability improves for many individuals who lost it during acute COVID-19, the prevalence of parosmia and phantosmia increases substantially over time. Olfactory dysfunction is associated with broader persistent symptoms of COVID-19, and may last for many months following acute COVID-19. Taste loss in the absence of smell loss is rare. Persistent qualitative smell symptoms are emerging as common long-term sequelae; more research into treatment options is strongly warranted given that even conservative estimates suggest millions of individuals may experience parosmia following COVID-19. Healthcare providers worldwide need to be prepared to treat post COVID-19 secondary effects on physical and mental health.

Figure 1.

Summary of participants described in the current study. As shown in the exclusion box, the majority of S2 respondents were excluded from present analyses due to inconsistent reports of their COVID-19 diagnosis between S1 and S2. Participants were also excluded for missing or inconsistent data, chemosensory dysfunction prior to COVID-19.

Figure 2.

A: Proportions of participants with smell, taste, and combined smell and taste impairments during baseline (S1, dark gray) and follow-up (S2, lighter grays and white). B: Proportions of qualitative smell changes at S1 (pink) and S2 (green), across all participants. C: Same analysis as B but stratified by individuals who later regained smell ability (white fill) or exhibited smell long-hauling (solid fill).

Figure 3.

A: Comparison of overall number of non-chemosensory symptoms at baseline (S1, pink) and follow-up (S2, green), stratified by smell long-hauler (LH) status at S2 with white fill indicating non-long haulers and solid fill indicating long-haulers. B: Comparison of selected symptoms at S1 and S2 stratified by smell long-haulers status at S2. C: Percentage of rare symptoms spontaneously mentioned in free text responses in English, Spanish, Dutch, Italian, and French.

Figure 4.

Distribution of ratings for smell ability at baseline (S1), stratified by whether a participant was classified as a smell long-hauler (pink) or non long-hauler (green) at follow-up (S2). In the original survey (i.e., baseline), the majority of both groups (i.e., more than 50%) reported complete smell loss (a score of zero on a VAS, shown on the x-axis); however, a greater proportion of those who would later become long haulers reported almost complete loss at S1, and fewer of the non long haulers reported near total loss at S1. The dashed vertical line indicates the smell ability rating (on a VAS from 0–100) where the two groups differ maximally.