ALXN1210-NMO-317

Treatment tested

• Ravulizumab

  Complement C5 inhibitor

Other measures (6)

Inclusion: - 1. Participant must be ≥ 2 to < 18 years of age at the time of signing the informed consent/assent. 2. Participants must be anti-AQP4 Ab-positive and have a diagnosis of NMOSD as defined by the 2015 international consensus diagnostic criteria. 3. Complement inhibitor treatment-naïve participants must have had at least 1 attack or relapse in the last 12 months prior to the Screening Period. For participants who are on off-label eculizumab, must have had at least 1 attack or relapse in the 12 months prior to first dose of eculizumab. 4. Expanded Disability Status Scale (EDSS) score ≤ 7 5. Eculizumab-experienced participants must be clinically stable per Investigator for 30 days and have been treated with eculizumab for at least 90 days prior to screening with no missed doses within 2 months prior to Day 1 and no interruptions in treatment since start of eculizumab. 6. 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. 7. To reduce the risk of meningococcal infection (Neisseria meningitidis), all participants must be vaccinated against meningococcal infection from serogroups A, C, W 135, and Y within 3 years prior to, or at least 14 days prior to Day 1, according to national/local guidelines. Participants who do not meet this requirement will be vaccinated against meningococcal infection according to national/local guidelines and will receive prophylactic antibiotics for at least 2 weeks after meningococcal vaccination if Day 1 occurs < 2 weeks after initial vaccination. 8. 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: - 1. Known to be human immunodeficiency virus (HIV) positive 2. History of N meningitidis infection 3. Active systemic bacterial, viral, or fungal infection within 14 days prior to Day 1. 4. Hypersensitivity to ravulizumab, murine proteins or to one of the excipients of ravulizumab. 5. Use of rituximab within 3 months prior to Screening 1. 6. 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 7. Use of intravenous immunoglobulin (IVIg) or plasma exchange (PE) within 3 weeks prior to Screening. 8. 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. 9. Use of immunomodulatory therapies for multiple sclerosis within 3 months prior to Screening.

To evaluate the efficacy of ravulizumab in pediatric participants with NMOSD