As with the Sterne (veterinary) anthrax vaccine strain and the similar British anthrax vaccine (known as AVP), AVA lacks the capsule plasmid pXO2 (required for full virulence) and is composed chiefly of the anthrax protective antigen (PA) with small amounts of edema factor (EF) and lethal factor (LF) that may vary from lot to lot. Other uncharacterized bacterial byproducts are also present. Whether or not the EF and LF contribute to the vaccines efficacy is not known. AVA has smaller amounts of EF and LF than AVP. Adjuvant
AVA contains aluminium hydroxide (alhydrogel) to adsorb PA as well as to serve as an adjuvant (immune enhancer). As such it is believed to stimulate humoral, but not cell-mediated, immunity. Each dose of the vaccine contains no more than 0.83 mg aluminum per 0.5 mL dose. (This is near the allowed upper limit of 0.85 mg/dose.) It also contains 0.0025% benzethonium chloride as a preservative and 0.0037% formaldehyde as a stabilizer. The mechanism of action of the adjuvant is not entirely understood. Potency/immunogenicity
Vaccination of humans with AVA induces an immune response to PA. More than 1/3 of subjects develop detectable anti-PA IgG after a single inoculation; 95% do so after a 2nd injection; and 100% after 3 doses. The peak IgG response occurs after the 4th (6 month) dose. The potency of AVA vaccine lots is routinely determined both by the survival rates of parenterally challenged guinea pigs and their anti-PA antibody titres as measured by an enzyme linked immunosorbant assay (ELISA).
Licensure and limited occupational use (1970-1991)
In 1970, AVA was first licensed by the USPHS for protection against cutaneous anthrax to a state-owned facility operated by the Michigan Department of Public Health. In 1973, the US Food and Drug Administration (FDA) first published standards for making, using and storing AVA. In the mid-1980s, the FDA approved it specifically for two limited preventive indications: 1) individuals who may come in contact with animal products or high-risk persons such as veterinarians and others handling potentially infected animals; and 2) individuals engaged in diagnostic or investigational activities involving anthrax spores. In 1985, the FDA published a Proposed Rule for a specific product review of the AVA, stating that the vaccines "efficacy against inhalation anthrax is not well documented" (a statement quoted controversially many years later). For many years, AVA was a little known product considered to be safe for pre-exposure use in the US in at-risk veterinarians, laboratory workers, livestock handlers, and textile plant workers who process animal hair. In 1990, the State of Michigan changed the name of its original production plant facility to the Michigan Biologic Products Institute (MBPI) as it gave up state ownership and converted it to a private entity.