The impact of incomplete vaccination schedules on the magnitude and duration of protective antigen-specific IgG responses in recipients of the US licensed anthrax vaccine

Using a cross-sectional analysis design, we measured serum anti-protective antigen (PA) concentrations in individuals receiving six or fewer US licensed anthrax vaccinations. Samples were collected from 363 individuals with a mean of 29.6 ± 8.42 months after their last vaccination (range 3–57 months). An enzyme-linked immunosorbent assay (ELISA) developed and validated by the Centers for Disease Control and Prevention (CDC) was used to evaluate the range and status of anthrax vaccine-induced serum antibody concentrations. A significant correlation (r = 0.73, P ≤ 0.001) was found to exist between the number of vaccinations received and specific anti-PA immunoglobulin G (IgG) concentrations. We observed two discrete groups comprised of one to three doses (5.9–11.7 μg/ml) and four to six doses (26.2–30.2 μg/ml). These data indicate that anti-PA IgG is present at low but detectable levels after as few as two vaccinations (5.9 ± 6.43 μg/ml). These findings may have significance for anthrax vaccine recipients who are unable to complete the primary or full regimen with this licensed product.