Immunogenicity of Seven-Valent Pneumococcal Conjugate Vaccine Administered at 6, 14 and 40 Weeks of Age in South African Infants

Background: The high cost of pneumococcal conjugate vaccine (PCV) and local epidemiological factors contributed to evaluating different PCV dosing-schedules. This study evaluated the immunogenicity of seven-valent PCV (PCV7) administered at 6-weeks; 14-weeks and 9-months of age. Methods: 250 healthy, HIV-unexposed infants were immunized with PCV7 concurrently with other childhood vaccines. Serotype-specific anti-capsular IgG concentrations were measured one-month following the 1st and 2nd PCV-doses, prior to and two-weeks following the 3rd dose. Opsonophagocytic killing assay (OPA) was measured for three serotypes following the 2nd and 3rd PCV7-doses. Immunogenicity of the current schedule was compared to a historical cohort of infants who received PCV7 at 6, 10 and 14 weeks of age. Results: The proportion of infants with serotype-specific antibody >= 0.35 mu g/ml following the 2nd PCV7-dose ranged from 84% for 6B to >= 89% for other serotypes. Robust antibody responses were observed following the 3rd dose. The proportion of children with OPA >= 8 for serotypes 9V, 19F and 23F increased significantly following the 3rd PCV7-dose to 93.6%; 86.0% and 89.7% respectively. The quantitative antibody concentrations following the 2nd PCV7-dose were comparable to that after the 3rd -dose in the 6-10-14 week schedule. Geometric mean concentrations (GMCs) following the 3rd PCV7-dose were higher for all serotypes in this study compared to the historical cohort. Conclusions: The studied PCV7 dosing schedule induced good immune responses, including higher GMCs following the 3rd-dose at 9-months compared to when given at 14-weeks of age. This may confer longer persistence of antibodies and duration of protection against pneumococcal disease.