Anthrax vaccine adsorbed: Further evidence supporting continuing the vaccination series rather than restarting the series when doses are delayed

Whether to restart or continue the series when anthrax vaccine doses are missed is a frequent medical management problem. We applied the noninferiority analysis model to this prospective study comparing the Bacillus anthracis protective antigen (PA) IgG antibody response and lethal toxin neutralization activity at day 28 to the anthrax vaccine adsorbed (AVA) (Biothrax®) administered on schedule or delayed. A total of 600 volunteers were enrolled: 354 in the on-schedule cohort; 246 in the delayed cohort. Differences were noted in immune responses between cohorts (p < 0.0001) and among the racial categories (p < 0.0001). Controlling for covariates, the delayed cohort was non-inferior to the on-schedule cohort for the rate of 4-fold rise in both anti-PA IgG concentration (p < 0.0001) and TNA ED50 titers (p < 0.0001); as well as the mean log10-transformed anti-PA IgG concentration (p < 0.0001) and the mean log10-transformed TNA ED50 titers (p < 0.0001). Providing a missed AVA dose after a delay as long as 5-7 years, elicits anti-PA IgG antibody and TNA ED50 responses that are robust and non-inferior to the responses observed when the 6-month dose is given on-schedule. These important data suggest it is not necessary to restart the series when doses of the anthrax vaccine are delayed as long as 5 or more years.