Anamnestic Immune Response in 3- to 4-year-old Children Previously Immunized With 10-valent Pneumococcal Nontypeable Haemophilus influenzae Protein D Conjugate Vaccine as 2-dose or 3-dose Priming and a Booster Dose in the First Year of Life

BACKGROUND: Immunogenicity of 10-valent pneumococcal nontypeable Haemophilus influenzae protein d conjugate vaccine (PHiD-CV), administered as 2-dose or 3-dose priming followed by a booster dose, has been described previously. The present study evaluated immunologic memory following PHiD-CV vaccination according to these vaccination schedules. METHODS: A dose of PHiD-CV (to test anamnestic responses) was administered to 172 children at 36 to 46 months of age; 110 of them had previously been vaccinated with PHiD-CV according to 2 + 1 or 3 + 1 schedules (PHiD-CV [2 + 1] and PHiD-CV [3 + 1] groups) and 62 were unprimed age-matched controls. To measure immune responses before and 7 to 10 days after the PHiD-CV dose, 22F-inhibition enzyme-linked immunosorbent assay and opsonophagocytic activity (OPA) assay were used. RESULTS: Serotype-specific IgG geometric mean concentrations (GMCs) and OPA geometric mean titers increased substantially (from before to 7 to 10 days after the additional PHiD-CV dose) for all 10 vaccines and 2 cross-reactive serotypes (6A and 19A) in the children previously vaccinated with PHiD-CV, regardless of the vaccination schedule used. Antibody GMCs and OPA geometric mean titers after the administration of the PHiD-CV dose were markedly higher in both previously PHiD-CV-vaccinated groups than in the unprimed control group, clearly demonstrating prior induction of immunologic memory. Antiprotein D antibody GMCs had also increased substantially from before to 7 to 10 days after vaccination in all 3 groups, with higher antibody GMCs in the previously vaccinated groups than in the control group. CONCLUSION: PHiD-CV vaccination according to 2 + 1 or 3 + 1 schedules resulted in comparable anamnestic immune responses. These findings suggest that similar protective efficacy may be achieved with both the schedules.