• Is at least 18 years of age (applicable for 1st study phase) or is at least 2 years of age (applicable for 2nd study phase)

• Has confirmed diagnosis of HAE type I or II

• Has had at least 3 moderate or severe HAE attacks (excluding extremity attacks) in the last 3 months before the Screening Visit. For participants ≥2 and ≤12 years of age, has had at least 1 moderate or severe HAE attack (excluding extremity attacks) in the last 6 months before Screening Visit

• Has a documented congenital C1-INH functional activity \<50% with or without C1-INH deficiency and C4 antigen level below the laboratory reference range

• Participant or the participant's legally authorized representative(s) has signed informed consent (as required by local law), with the assent of participants legally capable of providing it, as applicable

• States willingness to comply with all study procedures and availability for the duration of the study

• If the participant is of childbearing potential (CBP), has a negative pregnancy test and must have been using a highly effective method of contraception and continue to do so until at least 2 weeks after their last dose (for both blinded and open-label doses of IMP). Not of CBP is defined as surgically sterilized (hysterectomy, bilateral oophorectomy) or who are postmenopausal (defined as women with no menses for 12 months without an alternative medical cause). Highly effective methods of contraception:

‣ Combined hormonal contraception (estrogens and progesterone) methods such as oral, implantable, intravaginal, injectable, or transdermal contraceptives at a stable dose for a minimum of 1 full cycle (hormonal contraceptives must inhibit ovulation) and for at least 4 weeks before screening

⁃ Progesterone only hormonal contraception associated with inhibition of ovulation (oral, injectable, implantable)

⁃ Intrauterine device

⁃ Intrauterine hormone-releasing system inserted at least 4 weeks before screening

⁃ Bilateral tubal ligation/occlusion or vasectomized partner (with surgical success confirmed by medical assessment) OR Agrees to abstain from heterosexual intercourse during study participation and to use a highly effective contraceptive (as described above) as backup if they become sexually active during the study. Abstinence is only acceptable if this is the participant's usual lifestyle. Periodic abstinence (calendar, symptothermal, post-ovulation methods), withdrawal (coitus interruptus), spermicides only, and lactational amenorrhea method are not acceptable methods of contraception

⁃ Note: If a participant of CBP has a positive or suspected positive urine pregnancy test within 72 hours prior to treatment, a serum pregnancy test will be required

⁃ Male participants must not plan to father a child or donate sperm for 90 days after their last dose of study drug (for both blinded and open-label doses of the IMP). However, there are no official contraception requirements for male participants during the study.

• Has confirmed QAT per definition criteria

• Has a swelling episode that is new and not the continuation of a previous HAE attack