Infants 501-1500g birth weight and <32 0/7 weeks gestational age were enrolled from nine NICHD Neonatal Research Network centers from 2004 to 2006. Enrollment was stratified by weight group to yield approximately 20 infants per 100g increments from 501-1500g birth weight whose primary PCV-7 series was initiated before 3 months and completed by 8 months after birth. The infants' primary providers gave PCV-7 vaccination at 2, 4, and 6 months after birth. Infants had a single 2-ml blood sample drawn 4-6 weeks following the third dose of PCV-7. Antibodies for each of the seven vaccine serotypes included in PCV-7 were measured by enzyme-linked immunosorbent assay. Children were followed until 18-22 months corrected age to assess survival, infection, and neurodevelopmental outcomes.