Effect of delayed anthrax vaccine dose on Bacillus anthracis protective antigen IgG response and lethal toxin neutralization activity

We describe the Bacillus anthracis protective antigen IgG antibody response and the B. anthracis lethal toxin neutralization activity to a delayed dose of anthrax vaccine adsorbed (AVA, BioThrax®) using validated assays. 373 individuals received 1, 2, or 3 priming doses, 18-24 months afterward, they received a delayed dose of AVA. Overall, 23.6% of subjects showed detectable anti-PA IgG before the boost, compared to 99.2% (P < 0.0001) 28 days after the boost. Geometric mean anti-PA IgG concentration (GMC) was 1.66 μg/mL before and 887.82 μg/mL after the boost (P < 0.0001). The proportion of individuals with four-fold increase in GMC following the boost ranged from 93.8% to 100%. Robust anti-PA IgG levels and B. anthracis lethal toxin neutralization activity are induced when an AVA dose is delayed as long as two years. These data support continuing with the vaccination schedule when a dose is delayed as long as two years rather than restarting the series.