ings, it is estimated that a recombinant vaccine made from yeast will be available in two to three years.

The HBsAg genome also has been incorporated into the vaccinia virus, which then expresses HBsAg on its surface (Smith et al., 1983). Studies in chimpanzees of the resulting vaccine, its safety and immunogenicity, and its ability to protect against challenge with wild HBV already have begun. The advantages of using vaccinia as a vehicle for immunization against hepatitis are presumed ease of production (no need for procedures to inactivate HBV or to remove extraneous immunogens), low cost (technologies already are in place for vaccine production), and the use of live replicating virus. Potential barriers to the use of this vaccine include: (1) the fact that vaccinia vaccine for smallpox prophylaxis has been abandoned worldwide—some countries might be reluctant to reintroduce it; (2) the inherent danger of potential side effects in individuals with dermatitis or unrecognized immune deficiencies; and (3) uncertainty about successful “takes” following administration and the effectiveness of booster doses.

Several laboratories have identified the major antigenic site of HBsAg, a hydrophilic sequence of 8 to 12 amino acids (Dreesman et al., 1982a; Gerin et al., 1983a; Gerin et al., 1983b; Hopp, 1981; Lerner et al., 1981). Polypeptides of various sequences have been synthesized and studies of immunogenicity in animals have begun. The major barrier to successful development of synthetic vaccines is the poor immune response to small polypeptides. Current investigations are directed at enhancing immunogenicity, either through structural modifications (e.g., the circular polypeptide developed by investigators at Baylor) or by attaching the peptide to larger carrier molecules and adjuvants. Whether or not these efforts will succeed and how long they will take remains to be seen. Most estimates place the development time at six to ten years.

It is anticipated that improved hepatitis B vaccines would be as effective as current vaccines (or more so for some populations) and that some barriers now deterring use of plasma-derived vaccines would no longer apply. Therefore, utilization should be better. It is expected that both providers and potential recipients in the high-risk groups will have a high perception of the risk of infection (a score of eight out of ten), that physicians will have a somewhat higher perception of the severity of the disease than will lay people (a score of eight versus seven out of ten), and that both groups will have high expectations of the vaccine’s efficacy (a score of nine out of ten). Some barriers to utilization may persist. A high cost would inhibit some from being vaccinated. Compliance is usually worse for vaccines targeted for adults rather than for children, and this barrier may remain unless educational programs can overcome it. The modest degree of difficulty involved in initiating and completing immunization (e.g., the possible need for pre-immunization testing for HBV markers) may remain a barrier for some individuals.