The present invention is directed to a vaccine formulation providing for the extended release of antigenic material over time. The release profile of different embodiments can be varied, allowing a single administration to establish an active immunity in an animal, substantially equivalent to separately administered sensitising and booster shots according to conventional techniques. Preferred embodiments are solid or fluid based, and biodegradable. Variations allow for the periodic and/or sequential release of different antigenic material. Vaccines addressing members of the genus clostridium and their toxoids are specifically discussed.
