Background: Hereditary angioedema is a rare and potentially life-threatening genetic disease that is associated with kallikrein-kinin system dysregulation. Garadacimab (CSL312), a novel, fully-human monoclonal antibody that inhibits activated factor XII (FXIIa), is being studied for the prevention of hereditary angioedema attacks. The aim of this study was to evaluate the efficacy and safety of once-monthly subcutaneous administrations of garadacimab as prophylaxis for hereditary angioedema., Methods: VANGUARD was a pivotal, multicentre, randomised, double-blind, placebo-controlled, phase 3 trial that recruited patients (aged ≥12 years) with type I or type II hereditary angioedema across seven countries (Canada, Germany, Hungary, Israel, Japan, the Netherlands, and the USA). Eligible patients were randomly assigned (3:2) to receive garadacimab or placebo for 6 months (182 days) by an interactive response technology (IRT) system. Randomisation was stratified by age (≤17 years vs >17 years) and baseline attack rate (1 to <3 attacks per month vs ≥3 attacks per month) for the adult group. The randomisation list and code were kept by the IRT provider during the study, with no access by site staff and funding representatives. All patients and investigational site staff, and representatives from the funder (or their delegates) with direct interaction with the study sites or patients, were masked to treatment assignment in a double-blind fashion. Randomly assigned patients received a 400-mg loading dose of subcutaneous garadacimab as two 200-mg injections or volume-matched placebo on day 1 of the treatment period, followed by five additional self-administered (or caregiver-administered) monthly doses of 200-mg subcutaneous garadacimab or volume-matched placebo. The primary endpoint was the investigator-assessed time-normalised number of hereditary angioedema attacks (number of hereditary angioedema attacks per month) during the 6-month treatment period (day 1 to day 182). Safety was evaluated in patients who received at least one dose of garadacimab or placebo. The study is registered with the EU Clinical Trials Register, 2020-000570-25 and ClinicalTrials.gov, NCT04656418., Findings: Between Jan 27, 2021, and June 7, 2022, we screened 80 patients, 76 of whom were eligible to enter the run-in period of the study. Of 65 eligible patients with type I or type II hereditary angioedema, 39 were randomly assigned to garadacimab and 26 to placebo. One patient was randomly assigned in error and did not enter the treatment period (no dose of study drug received), resulting in 39 patients assigned to garadacimab and 25 patients assigned to placebo being included. 38 (59%) of 64 participants were female and 26 (41%) were male. 55 (86%) of 64 participants were White, six (9%) were Asian (Japanese), one (2%) was Black or African American, one (2%) was Native Hawaiian or Other Pacific Islander, and one (2%) was listed as other. During the 6-month treatment period (day 1 to day 182), the mean number of investigator-confirmed hereditary angioedema attacks per month was significantly lower in the garadacimab group (0·27, 95% CI 0·05 to 0·49) than in the placebo group (2·01, 1·44 to 2·57; p<0·0001), corresponding to a percentage difference in means of -87% (95% CI -96 to -58; p<0·0001). The median number of hereditary angioedema attacks per month was 0 (IQR 0·00-0·31) for garadacimab and 1·35 (1·00-3·20) for placebo. The most common treatment-emergent adverse events were upper-respiratory tract infections, nasopharyngitis, and headaches. FXIIa inhibition was not associated with an increased risk of bleeding or thromboembolic events., Interpretation: Monthly garadacimab administration significantly reduced hereditary angioedema attacks in patients aged 12 years and older compared with placebo and had a favourable safety profile. Our results support the use of garadacimab as a potential prophylactic therapy for the treatment of hereditary angioedema in adolescents and adults., Funding: CSL Behring., Competing Interests: Declaration of interests TJC is a speaker for Pharming, CSL Behring, Takeda, Fresenius Kabi, and Grifols; has received research and consultancy grants from CSL Behring, Takeda, BioCryst, Ionis, Spark, BioMarin, Fresenius Kabi, and Grifols; and is on the medical advisory board for the US Hereditary Angioedema Association, Director of ACARE Angioedema Center at Penn State University, Hershey, and on the Board of Directors for the American Academy of Allergy, Asthma, and Immunology. AR received research grants as a principal investigator, speaker, and advisor for CSL Behring, Takeda-Shire, BioCryst, Pharming, Pharvaris, Ionis, and Shulov Innovative Science. HHL is a speaker for Pharming, CSL Behring, Takeda, and BioCryst and has received research and consultancy grants from CSL Behring, Takeda, BioCryst, Ionis, BioMarin, Pharming, and Phavaris. JSJ is a speaker for Takeda-Shire, CSL Behring, Teva, AstraZeneca, GSK, Sanofi Genzyme, and Regeneron and has received research funding or consultancy fees from CSL Behring, Takeda-Shire, BioCryst, Novartis, Genentech, AstraZeneca, Allakos, Fresenius Kabi, GSK, and Regeneron. JAB is a consultant, principal investigator, and speaker for CSL Behring, Takeda-Shire, Pharming, and BioCryst; is a consultant or principal investigator for KalVista, Biomarin, and Ionis; and is a consultant for Astria, ONO Pharmaceutical, Pharvaris, and Cycle Pharmaceuticals. HFa received research grants from CSL Behring, Takeda, and Pharming; has served as an advisor for CSL Behring, Takeda, Pharming, KalVista, ONO Pharmaceutical, and BioCryst; and has participated in clinical trials or registries for BioCryst, CSL Behring, Pharming, KalVista, Pharvaris, and Takeda. WHY has been a speaker and advisory board member and has received honoraria from CSL Behring, Takeda-Shire, Novartis, Sanofi Genzyme, and Merck; has received research grants from CSL Behring, Takeda-Shire, BioCryst, Pharming, Aimmune, DBV Technologies, Eli Lilly, Pharvaris, AstraZeneca, Novartis, GSK, Genentech-Roche, Amgen, Sanofi Genzyme, Regeneron, Galderma, AnaptysBio, Glenmark, ALK Pharma, Dermira, Ionis, and Celgene; serves as a medical advisor (volunteer) for HAE Canada, a patient organisation; and is a member of Angioedema Centers of Reference and Excellence. ESGS has received lecturing or advertising board fees from Amgen, Sanofi, Novartis, AstraZeneca, Esperion, Ionis-Akcea, and Merck, paid to their institution. IO has received honoraria or served as a consultant or participated in advisory boards for CSL Behring, Takeda-Shire, and Torii Pharmaceutical Company. RTa is a speaker for BioCryst, CSL Behring, Pharming, AstraZeneca, Sanofi-Regeneron, GSK, and Takeda; has served as a consultant for BioCryst, CSL Behring, KalVista, Pharming, and Takeda; and has received grants or research support from BioCryst, CSL Behring, Ionis, KalVista, Pharvaris, and Takeda. MEM is a speaker for CSL Behring, Takeda, Pharming, BioCryst, GSK, Amgen, Sanofi-Regeneron, AstraZeneca, Blueprint, and Genentech; has received research grants from CSL Behring, Takeda, Pharming, BioCryst, KalVista, Pharvaris, GSK, Novartis, Merck, and Allakos; and has been a consultant for CSL Behring, Takeda, Pharming, BioCryst, KalVista, and Cycle Pharmaceuticals. WRL is a speaker for CSL Behring, Pharming, AstraZeneca, Sanofi-Regeneron, GSK, and Takeda-Shire; has served as a consultant for BioCryst, BioMarin, CSL Behring, Fresenius Kabi, Intellia, KalVista, Pharming, Pharvaris, and Takeda-Shire; is a board member of the US Hereditary Angioedema Association Medical Advisory Board; and has received grants or research support from ALK Pharma, BioCryst, CSL Behring, Ionis, Gossamer, KalVista, Kedrion, Therapure, and Takeda-Shire. IMS has received honoraria, research funding, and travel grants from BioCryst, CSL Behring, Pharming, Octapharma, KalVista, and Takeda-Shire and has served as a consultant or participated in advisory boards for these companies. EA-P has received honoraria as a speaker or advisor or grant support or clinical trial investigator support from BioCryst, BioMarin Europe, Centogene, CSL Behring, KalVista, Pharming, Pharvaris, and Takeda-Shire. BR has been a speaker and advisory board member for CSL Behring and Takeda, but has not received personal reimbursement for these activities. BR has participated in multiple clinical trials involving investigational drugs for CSL Behring, Takeda, BioCryst, Dyax, and Pharming, but does not hold patents or investments with these companies or involving this product, and serves as a volunteer Medical Scientific Advisor to HAE Canada, a patient organisation. GLS has been an advisory board member for CSL Behring and has participated in clinical trials for investigational drugs for CSL Behring, Takeda, BioCryst, Dyax, Pharming, Pharvaris, and KalVista. JA is a speaker bureau member for CSL Behring, Pharming, BioCryst, and Takeda and has received consulting fees from; is a clinical trial investigator for BioCryst, CSL Behring, Pharming, Takeda, KalVista, Pharvaris, and BioMarin; and has received consulting fees from Cycle Pharmaceuticals. KK has been a clinical trial investigator for CSL Behring. YS has received speaker fees from Novartis, AstraZeneca, Takeda-Shire, and Torii Pharmaceutical Company and has been a clinical trial investigator for CSL Behring. PS has received honoraria, research funding, travel grants, or has served as a consultant or participated in advisory boards for CSL Behring, Octapharma, Pharming, Shire, and Takeda. RTr has received honoraria, travel grants, or has participated in clinical trials or advisory boards for CSL Behring, Shire, and Takeda. HFe, FG, and IJ are full-time employees and shareholders of CSL Behring. MM has received financial support from CSL Behring for acting as a study centre investigator during the conduct of the study and personal fees from CSL Behring, Takeda-Shire, Pharming, BioCryst, Novartis, Octapharma, and KalVista outside the submitted work., (Copyright © 2023 Elsevier Ltd. All rights reserved.)