The APOLLO trial: a proof-of-concept study for vitamin A nasal drops in COVID-19-related postinfectious olfactory dysfunction.

Postinfectious olfactory dysfunction (PIOD) is common in COVID-19 patients. This 2-arm double-blinded randomized controlled trial (RCT) aimed to establish proof-of-concept for vitamin A versus placebo as a treatment modality for patients with PIOD. This study compared 9,000 IU daily self-administered vitamin A intranasal drops versus peanut oil drops over 12 wk in COVID-19 patients with PIOD. Outcome measures included: olfactory bulb volume (OBV), olfactory sulcus depth, cerebral functional MRI blood oxygen level dependent (BOLD) signal, Sniffin' Sticks TDI score, SSParoT, olfactory disorder questionnaire (ODQ) score, and brain-derived neurotropic factor (BDNF) levels were collected from participants at baseline and after trial intervention at 12 wk. Fifty-seven PIOD were recruited in the trial and allocated to vitamin A or placebo arm at a 2:1 ratio. After withdrawals and exclusions, 30 participants in the vitamin A arm and 15 in the placebo arm were analyzed. There was no significant difference in the change in OBV between both groups. Aside from an improvement in the quality-of-life component of ODQ questionnaire scores (P = 0.01), there were no significant differences in any of the other secondary outcome measures. This proof-of-concept trial has demonstrated no significant effect of intranasal vitamin A on olfactory function in COVID-19 PIOD patients. Further work is required to identify other therapeutic agents in the management of PIOD or evaluate a different PIOD cohort with non-COVID etiology.