Ravulizumab in Adult Participants With Paroxysmal Nocturnal Hemoglobinuria Currently Treated With High-Dose Eculizumab

Key Inclusion Criteria: 1. Documented diagnosis of PNH, confirmed by high-sensitivity flow cytometry evaluation of red blood cells and white blood cells, with granulocyte or monocyte clone size of ≥ 5%. 2. Received 1200 mg eculizumab every 12 to 16 days (every 2 weeks) for at least 3 months prior to Screening. 3. LDH ≤ 2 x upper limit of normal (ULN) according to central laboratory, at Screening. 4. To reduce the risk of meningococcal infection (Neisseria meningitidis), all participants must be vaccinated against meningococcal infections within 3 years prior to initiating study drug. 5. Body weight ≥ 40 kilograms. Key Exclusion Criteria: 1. History of major adverse vascular events within 6 months of Day 1. 2. History of bone marrow transplantation. 3. Lymphoma, leukemia, myelodysplastic syndrome, or any malignancy within the past 5 years except for basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years. 4. Concomitant use of anticoagulants is prohibited if not on a stable regimen for at least 2 weeks prior to Day 1. 5. Concomitant use of any of the following medications and not on a stable regimen (as judged by the Investigator) for the time period indicated prior to Screening: * Erythropoietin or immunosuppressants for at least 8 weeks * Systemic corticosteroids for at least 4 weeks * Vitamin K antagonists (for example, warfarin) with a stable international normalized ratio level for at least 4 weeks * Iron supplements or folic acid for 4 weeks 6. Live vaccine(s) within 1 month prior to Screening or plans to receive such vaccines during the study. 7. More than 1 LDH value \> 2 × ULN within the 6 months prior to Day 1. 8. Platelet count \< 30,000/cubic millimeter (30 × 10\^9/Liter \[L\]) at Screening. 9. Absolute neutrophil count \< 500/microliter (0.5 × 10\^9/L) at Screening.