The present study intends to investigate the use as a booster of a dose of sanofi pasteur's Pneumo 23 vaccine in the second year of life following 3-dose priming with Wyeth's Prevnar vaccine. The researchers will use a cohort of subjects who have received 3 doses of Prevnar® at 2, 4, 6 months of age in the context of the clinical study A3L12 on Hexavalent combined vaccine (DTaP-IPV-HB-PRP~T)