Safety, reactogenicity and immunogenicity of a recombinant protective antigen anthrax vaccine given to healthy adults

Bacillus anthracis causes anthrax, a vaccine-preventable zoonotic disease that may follow intentional or unintentional exposure to its spores. Although an anthrax vaccine is currently licensed in the USA, better vaccines are desirable for both pre- and post-exposure prophylaxis.

Healthy adults, aged 18-40 years, received anthrax immunization with either licensed Anthrax Vaccine Adsorbed (AVA, BioThrax), or an experimental recombinant Protective Antigen vaccine (rPA) produced from an avirulent, non-spore-forming strain of B. anthracis at one of four doses (5, 25, 50 or 75 microg). Volunteers were followed for safety, reactogenicity, and immunogenicity.

rPA vaccine was well tolerated with a low rate of local or systemic reactions. Although antibody responses were poor following unadjuvanted rPA administration, 89 and 100% of volunteers who received Alhydrogel-adjuvanted rPA given intramuscularly had four-fold increases by enzyme-linked immunosorbent and toxin neutralization assays, respectively. Peak antibody responses to adjuvanted rPA given intramuscularly were equivalent to AVA, given either intramuscularly or subcutaneously, when measured by either assay.

This recombinant Protective Antigen anthrax vaccine, when given with the adjuvant Alhydrogel to healthy adults in two intramuscular injections four weeks apart, is very well-tolerated and highly immunogenic.