Inclusion Criteria:

• Patient must be 18 years of age or older, at the time of signing the informed consent.
• Anti-aquaporin-4 antibody-positive and a diagnosis of NMOSD as defined by the 2015 international consensus diagnostic criteria.
• At least 1 attack or relapse in the last 12 months prior to the Screening Period.
  • NOTE: Patients with a single life-time attack will be considered to satisfy inclusion criterion #3 if the attack occurred in the last 12 months.
• Expanded Disability Status Scale score ≤ 7.
• Patients who enter the trial receiving supportive IST (eg, corticosteroids, azathioprine [AZA], mycophenolate mofetil [MMF], methotrexate [MTX], and tacrolimus [TAC]) for the prevention of relapse, either in combination or monotherapy, must be on a stable dosing regimen of adequate duration prior to Screening with no plan to change the dose during the study period (defined as from the screening visit through the end of the study) as follows: a. If patients who enter the study are receiving AZA, they must have been on AZA for ≥ 6 months and have been on a stable dose for ≥ 2 months prior to Screening. b. If patients who enter the study are receiving other ISTs (eg, MMF, MTX, or TAC), they must have been on the IST for ≥ 3 months and have been on a stable dose for ≥ 4 weeks prior to Screening. c. If patients who enter the study are receiving oral corticosteroids, they must have been on a stable dose for ≥ 4 weeks prior to Screening. d. If a patient enters the trial receiving oral corticosteroid(s) with or without other IST(s), the daily corticosteroid dose must be no more than prednisone 20 mg/day (or equivalent) prior to Screening.
• Vaccinated against N. meningitidis within 3 years prior to, or at the time of, initiating ravulizumab. Patients 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.
• Body weight ≥40 kilograms.
• Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
  • Male patients must agree to use contraception as detailed in the protocol during the treatment period and for at least 8 months after last dose of study drug and refrain from donating sperm during this period;
  • Not a woman of childbearing potential (WOCBP) OR • Is a WOCBP and using a highly effective or acceptable contraceptive method as described in Section 10.4 during the treatment period and for a minimum of 8 months after the last dose of study drug. − The investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study drug. A WOCBP must have a negative serum pregnancy test at screening and a negative urine pregnancy test before the first dose of study drug. Additional requirements for pregnancy testing during and after study drug. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy.
• Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol.

Exclusion Criteria:

• History of N. meningitidis infection.
• Human immunodeficiency virus (HIV) infection (evidenced by HIV-1 or HIV-2 antibody titer).
• History of unexplained infections.
• Active systemic bacterial, viral, or fungal infection within 14 days prior to study drug administration on Day 1.
• Presence of fever ≥ 38°C (100.4°F) within 7 days prior to study drug administration on Day 1.
• Hypersensitivity to murine proteins or to 1 of the excipients of ravulizumab.
• Any medical condition that, in the opinion of the Investigator, might interfere with the patient’s participation in the trial, poses any added risk for the patient, or confounds the assessment of the patient.
• Previously or currently treated with a complement inhibitor.
• Use of rituximab within 3 months prior to Screening.
• Use of mitoxantrone within 3 months prior to Screening.
• Use of Intravenous Immunoglobulin (IVIg) within 3 weeks prior to Screening.
• Participation in any other investigational drug study or exposure to an investigational drug or device within 30 days of Screening or 5 half-lives of the investigational drug, whichever is greater.
• Pregnant, breastfeeding, or intending to conceive during the course of the study.
• Patient is currently treated with a biologic medication 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, unless otherwise specified in the protocol.
• Participation in the PREVENT study (ECU-NMO-301), regardless of the study drug received (eculizumab or placebo).