Key Inclusion Criteria:  Diagnosis of paroxysmal nocturnal hemoglobinuria confirmed by a history of high-sensitivity flow cytometry from prior testing Treated with stable (ie, no change in dose or frequency) eculizumab therapy at the labeled dosing regimen or a higher dose and/or more frequently administered than labeled for at least 12 weeks prior to screening visit  Key Exclusion Criteria:  History of bone marrow transplantation or receipt of an organ transplant Body weight <40 kg at screening Current plans for modification of the following background concomitant medications, as applicable, during screening and treatment period: erythropoietin, immunosuppressive drugs, corticosteroids, anti-thrombotic agents, anticoagulants, iron supplements, and folic acid as described in the protocol Any use of complement inhibitor therapy other than eculizumab in the 12 weeks prior to the screening visit or planned use during the study Known hypocellular bone marrow based on a history of reduced age-adjusted bone marrow cellularity and/or bone marrow cellularity ≤25% No documented meningococcal vaccination within 5 years prior to screening visit unless it is documented that vaccination has been administered during the screening period and prior to initiation of study treatment Unable to take antibiotics for meningococcal prophylaxis, if required by local standard of care Any active, ongoing infection or a recent infection requiring ongoing systemic treatment with antibiotics, antivirals, or antifungals within 2 weeks of screening or during the screening period Documented positive polymerase chain reaction (PCR) or equivalent test based on regional recommendations for COVID-19 or suspected SARS-CoV-2 infection as described in the protocol Documented history of active, uncontrolled, ongoing systemic autoimmune diseases Recent, unstable medical conditions, excluding PNH and PNH-related complications, within the past 3 months prior to screening visit as described in the protocol Anticipated need for major surgery during the study  NOTE: Other protocol-defined Inclusion/ Exclusion Criteria apply.