In one embodiment, an expression system for expressing a UL128 complex is provided herein. The expression system may include a bacterial artificial chromosome (BAC) construct, wherein the BAC construct comprises a viral vector inserted with a set of DNA sequences that encode a UL128 complex. In another embodiment, a vaccine composition for preventing HCMV infection is provided. The vaccine composition may include a viral or bacterial vector capable of expressing a UL128 complex and a pharmaceutically acceptable carrier, adjuvant, additive or combination thereof or additional vector expressing a protein adjuvant. The viral vector may be an MVA and the UL128 complex includes five HCMV proteins or antigenic fragments thereof: UL128, UL130, UL131A, gL, and gH. In some embodiments, the viral vector is further inserted with one or more additional DNA sequences that encode one or more additional HCMVHCMV proteins or antigenic fragments thereof such as pp65, gB or both, or such as gM/gN or gO.