Key Inclusion Criteria:  1. Participants with PNH who are receiving treatment with pozelimab monotherapy in the R3918- PNH-1868 study (NCT04162470)  Key Exclusion Criteria:  Documented, positive polymerase chain reaction (PCR) or equivalent test based on regional recommendations for COVID-19 or suspected SARS-CoV-2 infection as defined in the protocol Participants with documented history of liver cirrhosis or participants with liver disease with evidence of currently impaired liver function; or participants with alanine aminotransferase (ALT) or aspartate aminotransferase (AST) as described in the protocol Significant protocol deviation(s) in the parent study based on the investigator's judgment as described in the protocol Any new condition or worsening of an existing condition which, in the opinion of the investigator, would make the participant unsuitable for enrollment or would jeopardize the safety of the participant Known hypersensitivity to cemdisiran or any component of cemdisiran formulation  NOTE: Other protocol-defined Inclusion/ Exclusion Criteria apply