COCOS-trial: Corticosteroids for COVID-19 induced loss of Smell. A single-centered, double-blind, randomized, placebo-controlled trial.

Background Prednisolone has been suggested as a treatment for olfactory disorders after COVID-19, but evidence is scarce. Hence, we aimed to determine the efficacy of a short oral prednisolone treatment on patients with persistent olfactory disorders after COVID-19. Methods We performed a randomized, double-blind, placebo-controlled, single-centered trial in the Netherlands. Patients were included if they were >18 years old and if they had persistent (>4 weeks) olfactory disorders within 12 weeks after a confirmed COVID-19 test. The treatment group received oral prednisolone 40mg once daily for ten days and the placebo group received matching placebo. In addition, all patients performed olfactory training. The primary outcome was the objective olfactory function on Sniffin’ Sticks Test (SST) 12 weeks after the start of treatment, measured in Threshold-Discrimination-Identification (TDI) score. Secondary outcomes were objective gustatory function assessed by Taste Strips (TST) and subjective self-reported outcomes on questionnaires about olfactory, gustatory and trigeminal function, quality of life, and nasal symptoms. Results Between November 2021 and February 2022 we included 115 eligible patients, randomly assigned to the treatment (n=58) or placebo group (n=57). No difference in olfactory function between groups was obtained after 12 weeks. Median TDI score on SST was 26∙8 (IQR 23∙6-29∙3) in the placebo group and 28∙8 (IQR 24∙0-30∙9) in the prednisolone group, with a median difference of 2∙0 (95% CI 0∙75 to 1∙5). There was similar improvement on olfactory function in both groups after 12 weeks. Furthermore, on secondary outcomes, we obtained no differences between groups. Conclusion This trial shows that prednisolone does not improve olfactory function after COVID-19. Therefore, we recommend not prescribing prednisolone for patients with persistent olfactory disorders after COVID-19.