Psychophysical assessment of olfactory and gustatory function in post-mild COVID-19 patients: A matched case-control study with two-year follow-up

Background: The aim of this study was to psychophysically evaluate the prevalence of smell and taste dysfunction two years after mildly symptomatic SARS-CoV-2 infection compared to that observed at one-year follow-up and while considering the background of chemosensory dysfunction in the no-COVID-19 population. Method: This is a prospective case-control study 93 patients with PCR-positive SARS-CoV-2 infection and 93 matched controls. Self-reported olfactory and gustatory dysfunction was assessed by Sino-nasal-Outcome-Test-22, item "Sense of smell or taste". Psychophysical ortho- and retronasal olfactory function and gustatory performance were estimated using the extended Sniffin' Sticks test battery, 20 powdered tasteless aromas, and taste strips test, respectively. Nasal trigeminal sensitivity was assessed by sniffing a 70% solution of acetic acid. Results: The two psychophysical assessments of chemosensory function took place after a median of 409 days (range: 366-461) and 765 days (range: 739-800) from the first SARS-CoV-2 positive swab, respectively. At two-year follow-up, cases exhibited a decrease in the prevalence of olfactory (27.9%% vs 42.0%; absolute difference, -14.0%; 95% CI, -21.8% to -2.6%; p = 0.016) and gustatory dysfunction (14.0% vs 25.8%; absolute difference, -11.8%; 95% CI, -24.2% to 0.6%; p = 0.098). Subjects with prior COVID-19 were more likely than controls to have an olfactory (27.9% vs 10.8 %; absolute difference, 17.2%; 95% CI, 5.2% to 28.8%) but not gustatory dysfunction (14.0% vs 9.7%; absolute difference, 4.3%; 95% CI, -5.8% to 14.4% p = 0.496) still two years after the infection. Overall, 3.2% of cases were still anosmic two-year after the infection. Conclusions: While a proportion of subjects recovered from long-lasting smell/taste dysfunction more than one year after COVID-19, cases still exhibited a significant excess of olfactory dysfunction two years after SARS-CoV-2 infection when compared to matched controls.