Efficacy and Safety Study of Ravulizumab IV in Pediatric Participants With NMOSD

Inclusion Criteria: * Participants must be anti-AQP4 Ab-positive and have a diagnosis of NMOSD as defined by the 2015 international consensus diagnostic criteria. * Complement inhibitor treatment-naïve participants must have had at least 1 attack or relapse in the last 12 months prior to the Screening Period. * Expanded Disability Status Scale (EDSS) score ≤ 7. * Eculizumab-experienced participants must be clinically stable per Investigator for 30 days and have been treated with eculizumab in Study ECU-NMO-303 for at least 90 days prior to screening with no missed doses within 2 months prior to Day 1. * Participants who enter the study receiving supportive IST(s) (eg, corticosteroid, azathioprine \[AZA\], mycophenolate mofetil \[MMF\], methotrexate \[MTX\], tacrolimus \[TAC\], cyclosporin \[CsA\], or cyclophosphamide \[CYC\]) for the prevention of relapse, either in combination or monotherapy, must be on a stable dosing regimen of adequate duration prior to Screening and remain on a stable dosing regimen during the Screening Period. * To reduce the risk of meningococcal infection (Neisseria meningitidis), all participants must be vaccinated against meningococcal infection. * Documented vaccination for Hib and S pneumoniae at least 14 days prior to Day 1 according to national/local guidelines for the applicable age group. Exclusion Criteria: * Use of rituximab within 3 months prior to screening. * Currently treated with a biologic medications (other than eculizumab) that may affect immune system functioning, or has stopped treatment with a biologic medication that may affect immune system functioning, and 5 half lives of the medication have not elapsed by the time of the Screening Visit. * Use of intravenous immunoglobulin (IVIg) or plasma exchange (PE) within 3 weeks prior to Screening. * Participation in another investigational drug or investigational device study (other than Study ECU-NMO-303) within 5 half lives of that investigational product (if known) or 30 days before initiation of the first dose of study drug, whichever is longer. * Use of immunomodulatory therapies for multiple sclerosis within 3 months prior to Screening.