are clearly superior to the polysaccharide alone in children under three years of age. They induce antibody levels well over those considered to be protective in 100 percent of children over 9 months of age (Lepow and Gordon, 1984), and 90 percent of infants (Zahradnik and Gordon, 1984). Further studies on the acceptability of conjugate vaccines for general use are in progress. The lower age limit for induction of protective antibody levels by these vaccines remains to be precisely determined.
The other approach to conjugate vaccines is to couple small oligosaccharides, derived from the type b polysaccharide, to diphtheria toxoid (Anderson, 1983). These vaccines also are immunogenic in young children.
Neither conjugate vaccine appears to be reactogenic in children (King et al., 1981); however, additional studies are required to determine their overall acceptability. Preliminary studies suggest that concurrent use of the polysaccharide-protein conjugate vaccine with the DTP vaccine has been shown to enhance the immune response to the free toxoid (Zahradnik and Gordon, 1984).
In conclusion, the purified type b polysaccharide has been demonstrated to be effective in children over 18 months of age, but not useful in children under 18 months of age. The polysaccharide-protein and oligosaccharide-protein conjugate vaccines are clearly more immunogenic than the polysaccharide alone in younger children.
Predictions on the further development of a vaccine for H. influenzae type b appear in Chapter 5.
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 various vaccines are displayed together to facilitate comparison.
Parental recognition of H. influenzae type b illness, or meningitis, as a threat to their child is probably mistakenly low, hence, a low score has been assigned to the perceived risk of illness. In contrast, high scores have been assigned in the severity and vaccination benefits categories because meningitis is recognized as a severe condition (possibly with physician cueing), and there is a strong belief in the benefits of pediatric vaccines in general. The need for more than one dose if the vaccine is administered to infants or young children is probably the only major perceived barrier to obtaining vaccination, so a moderately low score appears in that category.