Recently, a novel swine-origin influenza A/H1N1 virus was identified as a significant cause of febrile respiratory illnesses in Mexico and the United States. It rapidly spread to many countries around the world, prompting the World Health Organization to declare a pandemic on June 11, 2009. Data from several cohorts in different age groups that received licensed trivalent seasonal influenza vaccines suggest that these vaccines are unlikely to provide protection against the new virus. Adults are more likely to have measurable levels of serum hemagglutination inhibition assay (HAI) or neutralizing antibody than are children. These data indicate the need to develop vaccines against the new H1N1 strain and suggest that different vaccine strategies (e.g., number of doses, need for adjuvant) may be appropriate for persons in different age groups. Based on clinical data from other novel influenza A viruses, a higher dose, or multiple doses of an unadjuvanted, inactivated influenza H1N1 vaccine may be necessary to confer protection to a maximal number of vaccine recipients. This protocol explores the antibody response following vaccination at 2 different dosage levels (15 mcg and 30 mcg) in up to 650 children aged 6 months to 17 years, inclusive. This study assesses the immune response following a single dose of vaccine, to assess whether individuals have any pre-existing 'prime' immunity, such that the initial H1N1 vaccination serves as a boost, thus conferring a more rapid time to protection and the need for fewer doses. Antibody responses will be assessed after a second dose. This is a randomized, double-blinded, Phase II study in healthy infants, toddlers, children, and adolescents and is designed to investigate the safety, reactogenicity, and immunogenicity of an inactivated influenza H1N1 virus vaccine. The primary objectives are safety, to assess the safety of the unadjuvanted, inactivated influenza H1N1 vaccine when administered at the 15 mcg or 30 mcg dose; and immunogenicity, to assess the antibody response following a single dose of unadjuvanted, inactivated influenza H1N1 vaccine, stratified by age of recipient, when administered at the 15 mcg or 30 mcg dose. The secondary immunogenicity objective is to assess the antibody response following two doses of unadjuvanted, inactivated influenza H1N1 vaccine, stratified by age of recipient, when administered at the 15 mcg or 30 mcg dose. There will be 3 age strata, each containing 200 subjects: greater than or equal to 6 months to less than 36 months, greater than or equal to 36 months to 9 years, and 10 - 17 years . Subjects will be randomized into 2 groups, with 300 subjects per group (100 per strata), to receive intramuscular inactivated influenza H1N1 vaccine at 15 mcg (Group 1) or 30 mcg (Group 2). The vaccine will be administered on Days 0 and 21. Following immunization, safety will be measured by assessment of adverse events (AEs) for 21 days following the last vaccination (Day 42 for those receiving both doses and Day 21 for those who do not receive the second dose); serious adverse events (SAEs) and new-onset chronic medical conditions through 7 months post first vaccination (Day 201); and reactogenicity to the vaccines for 8 days following each vaccination (Day 0-7). Immunogenicity testing will include HAI and neutralizing antibody testing on serum obtained on the day of each vaccination (prior to vaccination) and 21 days following the second vaccination (Day 42). For subjects aged 10-17 years, serum for antibody assays will also be obtained at Day 8-10 following each vaccination. However, for the greater than or equal to 6 months - less than 36 months and the greater than o
