and aluminium phosphate for commercial vaccines. These newer types are also customarily called "alum" in some contexts (distinguished in this article by capitalization). Al salts are the most commonly used adjuvants in human vaccines. Their adjuvant activity was described in 1926.

The precise mechanism of Al salts ("Alum") remains unclear but some insights have been gained. It was formerly thought that they function as delivery systems by generating depots that trap antigens at the injection