from Stanley A. Plotkin and Bodo Plachter writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:
Prenatal transmission of CMV is a frequent cause of mental retardation and hearing loss in children. Furthermore, infection with this virus is a severe threat to immunocompromised patients. Consequently, development of a vaccine to prevent CMV disease has been identified as a first rank medical priority. Goals and target populations for such a vaccine have been identified. Antigens to be targeted by vaccine-induced immune responses have been defined indicating that only a subset of the more than 150 viral proteins may be sufficient to induce protective immunity. Using this information, strategies for the development of live virus vaccines as well as subunit vaccines have been developed. At this point at least seven candidate vaccines have been tested clinically and many other approaches are being explored.
Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention