vaccine. For the GBS vaccine candidate, the predicted efficacy is 0.80. Thus, the potential reduction in morbidity and mortality for the GBS vaccine is 2,856 using the committee median perspective and 2,823 using the age-neutral perspective. These values are not adjusted for vaccine adverse effects or anticipated utilization. Use of PRMM values for comparing vaccines is described in Chapter 7.

The type-specific polysaccharide antigens were identified as the protective antigens many years ago by Lancefield (1972). More recently, the sialated polysaccharide antigens have been purified and characterized by Jennings et al. (1980, 1983a, 1983b). Because the polysaccharide antigens may vary somewhat in molecular size, it may be possible to develop more immunogenic antigens consisting of conjugates or other larger polymers. The purified sialated polysaccharide antigens have been utilized in clinical trials to demonstrate their immunogenicity. A pilot study of GBS III vaccine in pregnant women recently began in Houston and is expected to be completed in late 1984 (Baker, personal communication, 1984). Success of maternal immunization is dependent on the production of the IgG isotype of antibodies that cross the placenta rather than IgM. Hence, candidate vaccines should be assessed for the classes of antibody they produce as well as for maternal immunogenicity.

Active immunization of the neonate probably would not be beneficial because the majority of cases of early-onset disease result from infection prior to delivery. Also, very young infants do not respond predictably to polysaccharide antigens, even when they are conjugated.

If active immunization of the mother is not possible, then passive immunization of the newborn could be carried out using an immunoglobulin reagent with known quantities of antibody to the GBS serotypes, prepared from immunized or immune donors (Santos et al., 1981; Vogel et al., 1980). If the studies in Houston referred to above confirm that immunization in pregnant women is effective and without significant side effects, then clinical trials could be carried out to demonstrate efficacy in prevention of neonatal early- and late-onset GBS disease. These clinical trials would require very large populations because of the relatively low incidence of the disease. Hence, they may require the collaboration of a number of centers or groups.

The health belief model parameters (perceptions of risk of illness, severity, vaccination benefits, and barriers) used to predict vaccine utilization are described in Chapter 6, where scores assigned to the vaccines are displayed together for comparison.