Abstract: A vertebrate animal, such as a human, is immunized with a bacterial host cell harboring a recombinant replicon which provides a stochastically expressed cell killing function whereby the cells are biologically contained. The replicon also comprises a gene encoding an antigen which is to be displayed on the outer surface of the host cell, so that it can elicit an immune response from the immunized animal. This antigen comprises one or more epitopes from a pathogenic agent.