Anthrax is a rare but critical infection caused by the bacterium Bacillus anthracis. Anthrax Vaccine Adsorbed (AVA) is the FDA-approved vaccine for anthrax infection, but its immunogenicity is still controversial. Raxibacumab is a human monoclonal antibody intended for the treatment of inhaled anthrax. The objective of this study is to assess the effect of raxibacumab on the immunogenicity of AVA.

This open-label, randomized, parallel-group, non-inferiority study included a total of 573 healthy participants (aged 18-65 years) with no previous exposure to protective antigen. The participants were randomly assigned in a 1:1 ratio to receive subcutaneous 0·5 mL AVA on days 1, 15, and 29 (n=276, or intravenous raxibacumab (40 mg/kg) before AVA on day-1, followed by subcutaneous 0·5 mL AVA on days 15 and 29 (n=269). The primary outcome of the study was the geometric mean concentration (GMC) ratio of anti-protective antigen antibodies between AVA alone and raxibacumab plus AVA.

After four weeks, the GMC of anti-protective antigen antibodies was higher in AVA alone (26.5 μg/mL) compared with raxibacumab plus AVA (22.5 μg/m). The GMC ratio between the two groups was 1.18. The risk of adverse events was similar in the two groups (20% vs. 29%).

The research concluded that the co-administration of 22 raxibacumab with AVA was non-inferior to AVA alone in the prevention of anthrax infection.