The present invention discloses an engineered bacteriophage capable of binding to a commensal bacterium and inserting its genome polynucleotide into the commensal bacterium, but incapable of producing progeny, incapable of carrying out a lysogenic cycle and incapable of carrying out a lytic cycle within the commensal bacterium, wherein the engineered bacteriophage comprises a genome polynucleotide including at least one gene encoding at least one heterologous antigen(s) under the control of a promoter.