RESUMEN
BACKGROUND: Annualized relapse rate (ARR) is used as an outcome measure in multiple sclerosis (MS) clinical trials. Previous studies demonstrated that ARR has reduced in placebo groups between 1990 and 2012. This study aimed to estimate real-world ARRs from contemporary MS clinics in the UK, in order to improve the feasibility estimations for clinical trials and facilitate MS service planning. METHODS: A multicentre observational, retrospective study of patients with MS from 5 tertiary neuroscience centres in the UK. We included all adult patients with a diagnosis of MS that had a relapse between 01/04/2020 and 30/06/2020. RESULTS: One hundred thirteen out of 8783 patients had a relapse during the 3-month study period. Seventy-nine percent of the patients with a relapse were female, the mean age was 39 years, and the median disease duration was 4.5 years; 36% of the patients that had a relapse were on disease-modifying treatment. The ARR from all study sites was estimated at 0.05. The ARR for relapsing remitting MS (RRMS) was estimated at 0.08, while the ARR for secondary progressive MS (SPMS) was 0.01. CONCLUSIONS: We report a lower ARR compared to previously reported rates in MS.