Haemophilus influenzae
vaccine containing polyribosyl
ribitol phosphate (PRP) or PRP covalently linked to
diphtheria toxoid (
PRP-D) was given to 94 healthy infants 17 to 22 months of age at the same time, but not at the same site, as a
diphtheria-
tetanus-
pertussis booster. Systemic reactions were similar in the two
vaccine groups and resembled those expected with the
diphtheria-
tetanus-
pertussis injection alone. Six (13%) and seven (14%) of the PRP and
PRP-D recipients, respectively, had minor local reactions to the
Haemophilus vaccine. Among the 77 children who were not already naturally immune (ie, anti-PRP antibody concentration of less than or equal to 0.15 micrograms of
protein per milliliter) before vaccination,
PRP-D was significantly more effective than PRP in inducing protective levels of antibody. Only 15 (43%) of the 35 nonimmune PRP recipients achieved a concentration of greater than or equal to 0.15 microgram/mL and only seven (20%) reached a concentration greater than or equal to 1.0 micrograms/mL following vaccination. In contrast, 34 (81%) of the 42 nonimmune recipients of
PRP-D had a concentration of greater than or equal to 0.15 microgram/mL following
vaccine and 32 (62%) had a concentration of greater than or equal to 1.0 micrograms/mL (P less than or equal to .001). These results suggest that more than one-half of nonimmune 18-month-old infants will not respond to PRP with protective levels of antibody. In light of the current data, recommendation for revaccination at 24 months of age for those immunized at any younger age is appropriate.