A Study of the Safety and Activity of Eculizumab in Pediatric Participants With Relapsing Neuromyelitis Optica Spectrum Disorder

Inclusion Criteria: 1. Male or female participants aged 2 years to \< 18 years with body weight ≥ 10 kilograms (kg). 2. Vaccinated against Neisseria meningitidis within 3 years prior to, or at the time of initiating eculizumab. Participants who initiate study drug treatment less than 2 weeks after receiving a meningococcal vaccine must receive appropriate prophylactic antibiotics until 2 weeks after the vaccination. 3. Documented vaccination against haemophilus influenzae type b and streptococcus pneumoniae infections at least 2 weeks prior to dosing as per local and country-specific immunization guidelines for the appropriate age group. 4. Anti-aquaporin-4 antibody-positive and diagnosis of NMOSD as defined by the 2015 International Panel for Neuromyelitis Optica Diagnosis criteria. 5. Historical Relapse Rate of at least 2 relapses in the last 2 years, and with at least 1 relapse in the year prior to Screening. 6. EDSS score ≤ 7. 7. Participants who enter the study receiving supportive immunosuppressive therapies (ISTs) for the prevention of relapse, either in combination or monotherapy, must be on a stable dosing regimen of adequate duration. 8. Female participants of childbearing potential must have a negative pregnancy test (serum human chorionic gonadotropin) at Screening and follow protocol-specified contraception guidance for avoiding pregnancy while on treatment and for 5 months after the last dose of eculizumab. 9. Male participants with a female spouse/partner of childbearing potential or a pregnant or breastfeeding spouse or partner must agree to use double barrier contraception (male condom plus appropriate barrier method for the female partner) while on treatment and for at least 5 months after the last dose of eculizumab. Exclusion Criteria: 1. Parent or legal guardian is an Alexion employee. 2. Pregnant, breastfeeding, or intending to conceive during the course of the study. 3. Participants known to be human immunodeficiency virus positive or with congenital immunodeficiency. 4. Unresolved meningococcal or other serious infection. 5. Any unresolved acute or chronic systemic bacterial or other infection that is clinically significant in the opinion of the Investigator and has not been treated with appropriate antibiotics. 6. Use of rituximab or other biologicals such as tocilizumab within 6 months prior to Screening. 7. Use of mitoxantrone within 3 months prior to Screening. 8. Use of intravenous immunoglobulin or plasma exchange within 3 weeks prior to Screening. 9. Use of immunomodulatory therapies for multiple sclerosis within 3 months prior to Screening. 10. Has previously received treatment with eculizumab or other complement inhibitors.