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How effective is Anthrax vaccine?

Updated December 29, 2023

Biothrax anthrax vaccine 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 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 Biothrax vaccine's effectiveness in 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. 

Efficacy studies of the use of CYFENDUS anthrax vaccine in humans have not been conducted due to ethical concerns. The efficacy of the vaccine was studied in rabbits and in non-human primates and the findings were used to predict the efficacy in humans. Animal studies were also conducted in guinea pigs to determine whether CYFENDUS improved survival rates after the completion of the recommended antibiotics in comparison to treatment with only antibiotics. According to the manufacturer, CYFENDUS increased survivor rates in guinea pigs in the 21 days after completion of antibiotics.     

IMPORTANT NOTE: NVIC “Quick Facts” is not a substitute for becoming fully informed about anthrax and the anthrax vaccine. NVIC recommends consumers read comprehensive information NVIC provides on anthrax, the vaccine manufacturer product information inserts, and speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child.

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