Capsular polysaccharide of Haemophilus influenzae type b as a vaccine.

The inability of infants to respond to Hib PRP with the production of protective antibodies is probably in large part a consequence of the relative immaturity of their immune system. It has been reported that PRP-induced antibodies in adults are primarily of the IgG subclass 2 and that there is a higher frequency of invasive H. influenzae type b diseases among adults with limited antibody deficiencies when the IgG subclass 2 isotype is not present at normal levels. There is evidence that adult levels of IgG subclass 2 are attained later in life than are adult levels of the other subclasses. Regulation of the level of this IgG subclass will undoubtedly be an important determinant in the induction of anti-PRP antibodies in infants. It seems unlikely that simple modification of PRP, such as by increasing molecular size, would result in an increased effectiveness in younger infants, because this group is inherently unable to synthesize subclass 2 antibodies. Currently intensive efforts are being made in several laboratories to develop vaccines that conjugate PRP with proteins that are capable of eliciting a T cell-dependent response in younger children. Preliminary results of several ongoing clinical trials of vaccines of PRP conjugated with diphtheria and with tetanus toxoids indicate that these vaccines are efficacious in children as young as 7 months of age. Researchers are hopeful that vaccines can be developed to protect infants ages 2 to 3 months.