An adverse event (AE) was defined as any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have to have a causal relationship with this treatment. TEAEs was defined as AEs that developed or worsened during the on-treatment period. SAE was defined as any untoward medical occurrence that resulted in any of following outcomes: death, life-threatening, required initial/prolonged in-participant hospitalization, persistent/significant disability/incapacity, congenital anomaly/birth defect/considered as medically important event. TEAEs included both Serious TEAEs and non-serious TEAEs.

SAF included all enrolled participants who received any study drug.

Percentage of participants who achieved LDH ≤1.5* Upper limit of normal (ULN) over Week 26, defined as LDH ≤1.5*ULN from baseline up to Week 26 were reported. A participant was considered to have met the criteria for adequate control of intravascular hemolysis if all of their LDH readings from the baseline through Week 26 inclusive or through the analysis end date, whichever is earlier, had values ≤ 1.5*ULN.

Full Analysis Set (FAS) included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure.

A participant was considered to have breakthrough hemolysis if he/she had any LDH measurement greater than or equal to (≥) 2*ULN, concomitant with associated signs or symptoms at any time subsequent to an initial achievement of disease control (i.e., LDH ≤ 1.5* ULN).

FAS included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure and "number analyzed" signifies those participants who were evaluable at the specified time points.

The overall rate of transfusion for a participant was calculated based on the duration of treatment exposure of the participant.

FAS included all enrolled participants who received any study drug.

Transfusion free was defined as not having received an RBC transfusion during the first 26 and 78 weeks. A transfusion was counted only if it was per-protocol, that is, if it follows the predefined transfusion algorithm: RBC transfusion due to a post-baseline hemoglobin level less than (<) 9 gram per deciliter (g/dL) (with anemia symptoms) or a post-baseline hemoglobin level < 7 g/dL (without anemia symptoms).

FAS included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure and "number analyzed" signifies those participants who were evaluable at the specified time points.

A participant was considered to have met the criteria for adequate control of intravascular hemolysis if all of his/her LDH readings from the baseline through Week 78 inclusive or through the analysis end date, whichever is earlier, had values <=1.5* ULN. and must not have discontinued study treatment early.

FAS included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure.

A participant was considered to have met normalization of intravascular hemolysis if all of their LDH readings from the baseline through Week 26 or 78 inclusive, or through the analysis end date, whichever is earlier, had values ≤ 1.0*ULN.

FAS included all enrolled participants who received any study drug. Here, "number analyzed" signifies those participants who were evaluable at the specified time points.

Change from baseline in LDH levels at Week 26, 78, and 104 was reported. Reported baseline is from R3918-PNH-1852 study.

FAS included all enrolled participants who received any study drug. Here, "number analyzed" signifies those participants who were evaluable at the specified time points.

Percent change from baseline in LDH levels at Week 26, 78, and 104 was reported. Reported baseline is from R3918-PNH-1852 study.

FAS included all enrolled participants who received any study drug. Here, "number analyzed" signifies those participants who were evaluable at the specified time points.

Change from baseline in RBC hemoglobin levels at Week 26, 78, and 104 was reported.

FAS included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure and "number analyzed" signifies those participants who were evaluable at the specified time points.

Change from baseline in free hemoglobin levels at Week 26, 78 and 104 was reported.

FAS included all enrolled participants who received any study drug. Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure and "number analyzed" signifies those participants who were evaluable at the specified time points.

Serum Concentrations of total REGN3918 was reported.

Here, "Overall number of Participants Analyzed" signifies participants who were evaluable for this outcome measure and "number analyzed" signifies those participants who were evaluable at the specified time points.

Number of Participants with treatment-emergent ADA response to REGN3918 was reported.

The ADA analysis set (AAS) includes all treated participants who received any amount of study drug (active [SAF]) and had at least 1 non missing anti pozelimab antibody result following the first dose of study drug. The AAS is based on the actual treatment received (as treated).

Here, "number analyzed" signifies those participants who were evaluable at the specified time points.