 Immunogenicity is assessed by assaying 16 serial blood samples obtained from all participants and a 17th sample from a subset of participants before vaccination and at other specified times. Total anti-protective antigen IgG antibody anti-PA IgG is quantified using a standardized and validated enzyme-linked immunosorbent assay ELISA; the primary study endpoints are 4-fold rise in antibody titer and antibody concentration relative to the pre-vaccination titers or assay reactivity threshold. A subset of serum samples is also assayed in an in vitro toxin neutralization assay TNA to measure the functional activity of anti-BioThrax antibodies. The kinetics of the immune response to BioThrax are examined at 3 time points in the study and blood samples from a subset of participants will be further tested in correlates of protection and immunogenetics substudies. All adverse events AEs, including vaccine reactogenicity, are actively monitored. While all AEs will be ascertained among study participants, several endpoints will be defined based on the likelihood of their occurrence andor their clinical importance. Of primary interest is the occurrence of local AEs such as warmth, tenderness, itching, pain, arm motion limitation, erythema, induration, nodule, and bruise. Systemic AEs such as fever, fatigue, muscle ache, headache, temperature, and painful axillary adenopathy are also evaluated. 