A timeline of recovery for post-COVID-19 olfactory dysfunction based on psychophysical performance: a scoping review and meta-analysis

Quantifying the prevalence of persistent COVID-19-associated olfactory dysfunction (C19OD) and estimating the timeframe for recovery of a functional sense of smell remains challenging. While self-reported recovery has notable limitations, psychophysical olfactory assessment provides a validated and standardized measure of olfactory status. To better understand C19OD prevalence, we conducted a meta-analysis using psychophysical olfactory data from studies of adults with previously confirmed SARS-CoV-2 infection. Our primary aim was to estimate the percentage of individuals who eventually recovered their sense of smell, defined as normosmia on psychophysical testing after at least 3 months of C19OD. Initial screening identified 583 abstracts, after which we conducted a full-text review of 151 studies, of which 64 comprising 9981 participants met the inclusion criteria. Estimates of recovery varied substantially by study design. Meta-regression analyses revealed that only longitudinal studies demonstrated a significant recovery trajectory over time, with predicted normosmia prevalence increasing from 46.0% at 3 months to 76.7% at 24 months post-infection. Cross-sectional meta-regression showed no significant association between time and normosmia prevalence in either recruitment frame. General post-COVID populations had greater prevalence of normosmia at 12–24 months (61.1%) than those with self-reported OD or long-COVID (30.2%), highlighting the disproportionate burden of persistent C19OD in these groups. The prevalence of C19OD is higher in studies using psychophysical testing than in earlier estimates based on self-report, which tends to underestimate deficits.