The primary purpose of this study is to evaluate the safety and efficacy of ravulizumab in pediatric participants with Neuromyelitis Optica Spectrum Disorder (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.

Alexion has a public commitment to allow requests for access to study data and will be supplying a protocol, CSR, and plain language summaries.