The purpose of this randomized, double-blind, placebo-controlled, parallel group study is to determine the efficacy of frexalimab in delaying the disability progression and the safety up to 36 months double-blind administration of study intervention compared to placebo in male and female participants with nrSPMS (aged 18 to 60 years at the time of enrollment). People diagnosed with nrSPMS are eligible for enrollment as long as they meet all the inclusion criteria and none of the exclusion criteria. Study details include: * This event-driven study will end when the target number of 6-month cCDP events is achieved, and the study is expected to last 43 months from randomization of the first participant to the common study end. * The number of scheduled visits will be up to 25 (including 3 follow-up visits) with a visit frequency of every month for the first 6 months and then every 3 months. * If the prespecified number of events for 6-month cCDP is not reached by V21/W180, scheduled visits will continue every 3 months.