Immunogenicity, safety, and lot consistency of the anthrax vaccine adsorbed, adjuvanted for post-exposure prophylaxis of anthrax in healthy adults: A phase 3, randomized, double-blind trial.

BACKGROUND

This double-blinded randomized Phase 3 study evaluated the immunogenicity and safety of CYFENDUS® vaccine (AV7909; Anthrax Vaccine Adsorbed, Adjuvanted) to support licensure for post-exposure prophylaxis following suspected or confirmed Bacillus anthracis exposure when administered with the recommended antibacterial drugs.

METHODS

Healthy adult participants (n = 3689) aged 18 to 65 were randomized to receive CYFENDUS vaccination (intramuscularly at 0, 2 weeks) or BioThrax® (Anthrax Vaccine Absorbed) [subcutaneously at 0, 2, 4 weeks]. Immunogenicity at Day 64 (seven weeks after the last CYFENDUS vaccine dose; five weeks after the last BioThrax vaccine dose) was evaluated using a 50 % neutralizing factor (NF50) threshold of protective immunogenicity generated by a toxin neutralizing antibody (TNA) assay, and by evaluating non-inferiority of CYFENDUS to BioThrax vaccination. Safety was assessed by physical exams, vital signs, solicited local injection site and systemic reactogenicity, and unsolicited adverse events (AEs).

RESULTS

The prospectively defined success criteria were met for the primary immunogenicity endpoints. The lower bound of the two-sided 95 % confidence interval (CI) for the proportion of CYFENDUS participants with TNA NF50 ≥ 0.56 was above the pre-defined criterion of ≥40 % (95 % CI: 64.5 %, 68.1 %). The lower bound of the two-sided 95 % CI of the difference in the proportion of participants with TNA NF50 ≥ 0.29 in the CYFENDUS versus the BioThrax vaccine group was greater than the pre-defined non-inferiority criterion of -15 % as well as demonstrating statistical superiority (95 % CI: 20.0, 29.2 %), a closed hypothesis test supported by regulatory agencies in well-controlled clinical trials. The most common adverse events (AE) were injection site-related; most reported solicited reactogenicities and AEs were either Grade 1 or 2 severity.

CONCLUSIONS

The study met the pre-defined endpoint criteria, demonstrating protective level of immune response and non-inferiority of CYFENDUS to BioThrax vaccination. The CYFENDUS vaccine was well-tolerated in healthy adults.

TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03877926.