To date in the United States over 3000 subjects under 2 years of age have received 1 or more doses of PRP-D vaccine. Thus far there have been no statistically significant differences between the rate of local or systemic reactions among recipients of this vaccine and that among those who received the PRP or placebo control vaccines. The vaccine is well-tolerated and is not associated with clinically significant adverse reactions. The PRP-D vaccine has been shown to be a T cell-dependent immunogen, inducing sequential increases in antibody level with repeated immunization and producing a high proportion of IgG relative to IgM. A recall response, or immunologic memory effect, has been demonstrated in children 1 year after receiving PRP-D. PRP-D is consistently immunogenic in all age groups. In adults this vaccine induces a geometric mean antibody response greater than 200 micrograms/ml. In infants 7 to 24 months of age levels greater than 1 microgram/ml have been achieved in over 90% of the subjects with 1 or 2 doses of PRP-D, whereas 90% or more of infants under 7 months of age achieve levels of 0.15 microgram/ml or greater.