Anthrax vaccine adsorbed, sold under the brand name Biothrax among others, is a vaccine intended to provide acquired immunity against Bacillus anthracis.

Anthrax vaccine adsorbed originated in studies done in the 1950s and was first licensed for use in humans in 1970. In the US, the principal purchasers of the vaccine are the Department of Defense and Department of Health and Human Services. Ten million courses (60 million doses) of the vaccine have been purchased for the US Strategic National Stockpile in anticipation of the need for mass vaccinations owing to a future bio-terrorist anthrax attack. The product has attracted some controversy owing to alleged adverse events and questions as to whether it is effective against the inhalational form of anthrax.

Anthrax vaccine adsorbed (Biothrax) is indicated for the active immunization for the prevention of disease caused by Bacillus anthracis in people aged 18 through 65 years of age. It is approved for both pre-exposure prophylaxis of disease in persons at high risk of exposure and post-exposure prophylaxis of disease following suspected or confirmed Bacillus anthracis exposure, when administered in conjunction with recommended antibacterial drugs.

Anthrax vaccine adsorbed, adjuvanted (Cyfendus) is indicated for post-exposure prophylaxis of disease following suspected or confirmed exposure to Bacillus anthracis in people aged 18 through 65 years of age when administered in conjunction with recommended antibacterial drugs.

Anthrax vaccine adsorbed was developed by Emergent Biodefense Operations.

Anthrax vaccine adsorbed is classified as a subunit vaccine that is cell-free, meaning it lacks any whole or live anthrax bacteria. The antigen (immunologically active) portions are produced from culture filtrates of a toxigenic, but avirulent, nonencapsulated mutant — known as V770-NP1-R — of the B. anthracis Vollum strain. (The Vollum strain was the same one weaponized by the old U.S. biological warfare program.) As with the Sterne (veterinary) anthrax vaccine strain and the similar British anthrax vaccine (known as AVP), anthrax vaccine adsorbed 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 across vaccine production lots. Other uncharacterized bacterial byproducts are also present. Whether or not the EF and LF contribute to the vaccine's efficacy is not known. Anthrax vaccine adsorbed has smaller amounts of EF and LF than AVP.

Anthrax vaccine adsorbed 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.

Vaccination of humans with anthrax vaccine adsorbed 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 anthrax vaccine adsorbed 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). The shelf-life of anthrax vaccine adsorbed is reported to be three years when stored between 2 and 8 °C (36 and 46 °F) and never frozen.