Inclusion Criteria:  Age at screening 2-11 years, inclusive Body weight at screening >=10 kg For female patients of childbearing potential (postmenarchal): agreement to either remain completely abstinent (refrain from heterosexual intercourse) or to use a reliable means of contraception Diagnosed as having NMOSD with AQP4 antibody seropositive status as defined by the Wingerchuk 2015 criteria Clinical evidence of at least one documented attack (including first attack) in the last year prior to screening Neurological stability for >=30 days prior to both screening and baseline Expanded Disability Status Scale (EDSS) 0 to 6.5 For patients receiving a baseline immunosuppressant treatment and planning to continue on these therapies, treatment must be at stable dose for 4 weeks prior to baseline  Exclusion Criteria:  Pregnancy or lactation Evidence of other demyelinating disease mimicking NMOSD Active or presence of recurrent bacterial, viral, fungal, mycobacterial infection, or other infection at baseline Evidence of chronic active hepatitis B or C Evidence of untreated latent or active tuberculosis (TB) Receipt of a live or live-attenuated vaccine within 6 weeks prior to baseline History of severe allergic reaction to a biologic agent