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The current anthrax vaccine, Biothrax, is given in a five-dose series over 18 months, followed by annual booster shots to maintain protection for those individuals who continue to be at risk for anthrax exposure. For persons who have completed the initial five doses and are not considered high risk but would like to maintain vaccine-acquired protection, ACIP recommends a booster dose every three years.
There is limited published data on controlled human trials to test the vaccine’s effectiveness. Developed primarily to protect workers exposed to skin anthrax by handling animals and animal by-products, the vaccine was tested in four field trials during the late 1950s. Vaccine researchers stated that while the vaccine appeared to be protective against cutaneous anthrax, the data was inconclusive on whether protection would extend to inhalation anthrax. Additionally, they reported that the vaccine did not offer long-term protection and that three primary vaccine doses and a booster dose were needed to ensure that the vaccine could protect against anthrax for at least six months. Based on one study published in 1962, which involved only 1,249 individuals, the vaccine was reported to be 92.5 percent effective against cutaneous anthrax.
There is little human data on the current vaccine's effectiveness against persons who get the rarer but more deadly form of inhalation anthrax. Experimental data in macaques and rabbits suggests the vaccine may be partially effective against inhalation anthrax; however, it was not fully effective in guinea pigs and mice.
The Biothrax vaccine package insert states that the vaccine may not protect all individuals. Individuals who are immunocompromised may have a decreased immune response to the vaccine.