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)[4] 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.