Response of 7- to 15-Month-Old Infants to Sequential Immunization With Haemophilus influenzae Type b-CRM197 Conjugate and Polysaccharide Vaccines

• Objective.  —To evaluate the immunologie potential of infants 7 to 15 months of age to respond to Haemophilus influenzae type b polysaccharide vaccine following immunization with H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine.

Study Design.  —One hundred seventy-one infants, aged 7 to 15 months, were consecutively and alternatively assigned to one of three immunization protocols. Group 1 (n = 71) received three doses of H influenzae b oligosaccharideCRM₁₉₇ conjugate vaccine, group 2 (n = 47) received two doses of H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine followed by one dose of H influenzae type b polysaccharide vaccine, and group 3 received one dose of H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine followed by two doses of H influenzae type b polysaccharide vaccine. Immunizations were given on day 0 and at 2 months and 6 months. Anti-H influenzae type b polysaccharide antibody levels were measured on day 0 and 2, 3,6, 7, and 12 months after the study began.

Results.  —Haemophilus influenzae type b polysaccharide vaccine given as a second dose stimulated an antibody rise but did so less effectively than H influenzae b oligosaccharideCRM₁₉₇ conjugate vaccine. Two doses of H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine were highly immunogenic; geometric means were 31 and 35 γg/mL in the 7to 11-month and 12- to 15-month age groups, respectively. Following two doses of H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine, both immunization protocols resulted in (1) equally high geometric mean antibody levels 1 month after immunization and (2) similar geometric mean antibody levels 6 months after immunization.

Conclusions.  —Haemophilus influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine induces antibody levels that would be expected to protect infants from initial invasion and primes the immune system for an anamnestic response. Our data indicate that if a booster immunization is needed, H influenzae type b polysaccharide vaccine could be an alternative to H influenzae b oligosaccharide-CRM₁₉₇ conjugate vaccine.(AJDC. 1991;145:898-900)