Background: Induction of immunologic memory was assessed following primary vaccination with 10-valent pneumococcal nontypeable Haemophilus influenzae protein D conjugate vaccine (PHiD-CV).
Methods: Infants were randomized (1:1) to receive 3 doses of PHiD-CV or 7vCRM (7-valent CRM197-conjugated pneumococcal conjugate vaccine [PCV]) at 2, 3, and 4 months of age followed by 23-valent pneumococcal polysaccharide vaccine (23vPS) booster dose at 11 to 14 months of age. Pneumococcal geometric mean antibody concentrations (GMCs) and opsonophagocytic activity (OPA) geometric mean titers were measured.
Results: Postprimary immune responses were consistent with those in previous PHiD-CV and 7vCRM studies. Following 23vPS boosting, vaccine serotype-specific antibody GMCs increased 6.5- to 33.3-fold and 4.8- to 32.2-fold versus prebooster in the PHiD-CV and 7vCRM groups, respectively. Postbooster OPA titers increased 2.8- to 38.8-fold and 2.6- to 58.9-fold, respectively. Postbooster antibody GMCs exceeded postprimary levels but, for some serotypes, postbooster OPA geometric mean titers were lower than postprimary in both groups. An additional dose of the same PCV received for priming was administered to 52 children aged 46 to 50 months, resulting in higher responses versus postprimary vaccination for all serotypes, but not always higher than post-23vPS booster.
Conclusions: Induction of immunologic memory following PHiD-CV priming was confirmed. Additional PCV boosting in 4-year-olds did not provide strong evidence of hyporesponsiveness induced by previous 23vPS boosting. However, our results did not rule out depletion of the memory B cell pool following 23vPS vaccination, resulting in subsequent attenuated immune responses, and therefore support the use of PCV rather than 23vPS for booster vaccination in the second year of life.