Donidalorsen Treatment in Children With Hereditary Angioedema

Key Inclusion Criteria: 1. Must be between the ages of 2 and less than 12 years, inclusive, at the time of informed consent and, as applicable, assent. 2. Must weigh at least 9 kg at the time of informed consent and, as applicable, assent. 3. Documented diagnosis of HAE-1/HAE-2 based upon both of the following: 1. Documented clinical history consistent with HAE (SC or mucosal, non-pruritic swelling episodes without accompanying urticaria). 2. Diagnostic testing results that confirm HAE-1/HAE-2: C1-inhibitor (C1-INH) functional level \<50% normal level AND complement factor C4 level below the lower limit of normal (LLN); OR a known pathogenic mutation in the SERPING1 gene. Key Exclusion Criteria: 1. Must not have any screening laboratory abnormalities or any other clinically significant abnormalities during screening that would render a participant unsuitable for inclusion. 2. Must not have been treated with another investigational drug, biological agent, or device within 1 month of Screening, or 5 half-lives of investigational agent, whichever is longer. 3. Concurrent diagnosis of any other type of recurrent angioedema, including idiopathic angioedema or HAE with normal C1-INH (HAE-nC1-INH or Type III). Note: Other protocol-specified inclusion/exclusion criteria may apply.