Inclusion Criteria: 1) Parent/guardian gives written informed consent. 2) Good health as determined by negative gestational history, subject's medical history and physical assessment. Medical history will include a review of the major body systems (e.g., ear-nose-throat, cardiovascular, respiratory, neurological, gastrointestinal, genitourinary, hematological). 3) Newborn cohort: within 3 days of birth and gestational age = 32 weeks and < 37 weeks and a birth weight = 2000 grams and < 3000 grams. 4) Able to attend the scheduled visits and to comply with the study procedures. Exclusion Criteria: 1) Infant history of vaccinations and date received is unknown or undocumented. 2) Known history of high-level mercury exposure in mother or infant. 3) Any contraindication to routine vaccinations. 4) Clinically significant findings on review of systems (determined by investigator or sub-investigator to be sufficient for exclusion). 5) Known or suspected impairment of immunologic function or receipt of immunosuppressive therapy or immunoglobulin since birth. 6) HIV-positive mother by history. 7) Personal or immediate family history of congenital immune deficiency. 8) Developmental delay or neurological disorders.