Key Inclusion Criteria:  Diagnosis of PNH confirmed by a history of high-sensitivity flow cytometry from prior testing Treated with eculizumab or ravulizumab prior to screening visit as described in the protocol Note: Biosimilars are not permitted, unless approved by the Sponsor  Key Exclusion Criteria:  Patients with a screening LDH >1.5 × ULN who have not taken their C5 inhibitor within the labeled dose interval at the dose prior to the screening LDH assessment Receipt of an organ transplant, history of bone marrow transplantation or other hematologic transplant Body weight < 40 kilograms at screening visit Any use of complement inhibitor therapy other than eculizumab or ravulizumab in the 26 weeks prior to the screening visit or planned use during the study with the exception of study treatments Not meeting meningococcal vaccination requirements for eculizumab or ravulizumab according to the current local prescribing information (where available) and at a minimum documentation of meningococcal vaccination within 5 years prior to screening visit. Any contraindication for receiving Neisseria meningitidis vaccination. Positive for hepatitis B, and/ or hepatitis C as described in the protocol History of cancer within the past 5 years, except for adequately treated basal cell skin cancer, squamous cell skin cancer, or in situ cervical cancer Participation in another interventional clinical study (except R3918-PNH-2021) or use of any experimental therapy within 30 days before screening visit or within 5 half-lives of that investigational product, whichever is greater, with the exception of eculizumab or ravulizumab. Patients with functional or anatomic asplenia  Note: Other protocol-defined Inclusion/ Exclusion Criteria apply