Background: Natalizumab 4-week dosing (Q4W) with 300 mg is approved for treatment of relapsing-remitting multiple sclerosis. Dosing frequency of approximately 6 weeks (Q6W) is associated with lower progressive multifocal leukoencephalopathy (PML) risk in retrospective analyses. Real-world data suggest similar effectiveness, but NOVA is the first randomized trial to assess Q6W efficacy. Objectives: Evaluate the efficacy of natalizumab Q6W in patients previously treated with natalizumab Q4W for ?12 months compared with continuation of Q4W over 72 weeks. Methods: NOVA is a randomized, controlled, open-label, rater-blinded phase 3b trial. Included patients were treated with natalizumab Q4W without relapse for ?12 months and had no enhancing lesions at screening. Patients were randomized 1:1 to Q6W or Q4W arms. The primary endpoint was number of new/newly enlarging T2 (N/NET2) hyperintense lesions analysed by negative binomial regression with baseline (BL) weight, prior natalizumab exposure, and region as covariates. Secondary endpoints included relapses and 24-week confirmed disability worsening (CDW). Primary estimand used all observed data; secondary estimand treated post-intercurrent event data as missing. Missing data were imputed by worst value on treatment or multiple imputation depending on discontinuation reason. Results: 195/248 (79%) Q4W patients and 207/251 (82%) Q6W patients completed NOVA. BL characteristics were well balanced. Proportions of patients with N/NET2 lesions among observed data were low in both arms (Q4W:4.1%; Q6W:4.3%). Mean N/NET2 lesions in the Q4W and Q6W arms with the primary estimand were 0.05 and 0.20 (P=0.0755) and 0.06 and 0.31 (P=0.0437) with the secondary estimand. The difference was mainly due to 2 Q6W patients with high values: (1) 30 lesions that occurred 3 months after natalizumab discontinuation and (2) 25 lesions that occurred with asymptomatic PML observed at week 72. The sum of all other N/NET2 lesions in NOVA was 18 with no other patient having >2. Relapse occurred in 2.1% and 2.8% (P=0.64) and CDW occurred in 8% and 10% (P=0.40) of patients in the Q4W and Q6W arms, respectively. Safety data were consistent with the known drug profile and similar between groups. Conclusions: Despite a small difference in efficacy between arms, NOVA data suggest the vast majority of patients stable on Q4W dosing can switch to Q6W dosing with no clinically meaningful loss of efficacy. No conclusions on PML risk can be drawn from NOVA.