• Number of Participants Reporting Solicited Subjective Local Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of local reactions of pain, tenderness and swelling for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Reporting Solicited Subjective Local Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of local reactions of pain, tenderness and swelling for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Reporting Solicited Quantitative Local Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of local reactions of swelling and redness for 8 days after vaccination (Day 0-7). If the reaction was present, the maximum diameter was measured in millimeters (mm). Participants are counted if they were reported as experiencing the reaction with any measurement greater than 0 mm on any of the 8 days.
  
  
• Number of Participants Reporting Solicited Quantitative Local Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of local reactions of swelling and redness for 8 days after vaccination (Day 0-7). If the reaction was present, the maximum diameter was measured in millimeters (mm). Participants are counted if they were reported as experiencing the reaction with any measurement greater than 0 mm on any of the 8 days.
  
  
• Number of Participants Reporting Solicited Quantitative Systemic Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily oral/axillary temperatures and the number of vomiting episodes, if experienced, for 8 days after vaccination (Day 0-7). Participants are counted as experiencing fever if they reported oral temperatures of 38.3 degrees Celsius or higher, or axillary temperatures of 37.8 degrees Celsius or higher, on any of the 8 days. Participants are counted as experiencing vomiting if they reported one or more episodes of vomiting on any of the 8 days.
  
  
• Number of Participants Reporting Solicited Quantitative Systemic Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily oral/axillary temperatures and the number of vomiting episodes, if experienced, for 8 days after vaccination (Day 0-7). Participants are counted as experiencing fever if they reported oral temperatures of 38.3 degrees Celsius or higher, or axillary temperatures of 37.8 degrees Celsius or higher, on any of the 8 days. Participants are counted as experiencing vomiting if they reported one or more episodes of vomiting on any of the 8 days.
  
  
• Number of Participants Age 6 to Less Than 36 Months Reporting Solicited Subjective Systemic Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants' parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of irritability, decreased appetite and lethargy for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Age 6 to Less Than 36 Months Reporting Solicited Subjective Systemic Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants' parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of irritability, decreased appetite and lethargy for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Age 36 Months to 9 Years Reporting Solicited Subjective Systemic Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of feverishness, myalgia, headache, nausea and decreased general activity for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Age 36 Months to 9 Years Reporting Solicited Subjective Systemic Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of feverishness, myalgia, headache, nausea and decreased general activity for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Age 10 to 17 Years Reporting Solicited Subjective Systemic Reactions After the First Vaccination [ Time Frame: Day 0-7 after first vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of feverishness, myalgia, headache, nausea, decreased general activity, and malaise for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Age 10 to 17 Years Reporting Solicited Subjective Systemic Reactions After the Second Vaccination [ Time Frame: Day 0-7 after second vaccination ] [ Designated as safety issue: Yes ]
  Participants or their parents/guardians maintained a memory aid to record daily the occurrence of systemic symptoms of feverishness, myalgia, headache, nausea, decreased general activity, and malaise for 8 days after vaccination (Day 0-7) based on their interference with daily activities. Participants are counted if they were reported as experiencing the symptom at any severity on any of the 8 days.
  
  
• Number of Participants Reporting Vaccine-associated Serious Adverse Events (SAEs) [ Time Frame: Day 0 through Day 180 after last vaccination ] [ Designated as safety issue: Yes ]
  Serious adverse events included any untoward medical occurrence that resulted in death; was life threatening; was a persistent/significant disability/incapacity; required in-patient hospitalization or prolongation thereof; resulted in a congenital anomaly/birth defect; or may have jeopardized the participant or required intervention to prevent one of these outcomes. Association to vaccination was determined by a study clinician licensed to make medical diagnoses.
  
  
• Number of Participants Age 6 to Less Than 36 Months With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 0 [ Time Frame: Day 0 ] [ Designated as safety issue: No ]
  Blood was collected from all participants prior to vaccination for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 6 to Less Than 36 Months With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 8-10 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 8-10 ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 6 to Less Than 36 Months With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 21 ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum, for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 36 Months to 9 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 0 [ Time Frame: Day 0 ] [ Designated as safety issue: No ]
  Blood was collected from all participants prior to vaccination for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 36 Months to 9 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 8-10 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 8-10 ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 36 Months to 9 Years Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 21 ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum, for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 10 to 17 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 0 and at Days 8-10 and 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to vaccination and Days 8-10 and 21 after first vaccination ] [ Designated as safety issue: No ]
  Blood was collected from all participants prior to vaccination and at Days 8-10 and 21 for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 6 to Less Than 36 Months With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 8-10 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to and Day 8-10 after first vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 6 to Less Than 36 Months With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to and Day 21 after first vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum, for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 21 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 36 Months to 9 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 8-10 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to and Day 8-10 after first vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 36 Months to 9 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to and Day 21 after first vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum, for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 21 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 10 to 17 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Days 8-10 and 21 Following a Single Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to and Days 8-10 and 21 after first vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from all participants enrolled in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 or Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 or Day 21 titer was an increase by 4-fold or more.
  
  

• Number of Participants Age 6 to Less Than 36 Months With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 6 to Less Than 36 Months With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 36 Months to 9 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 36 Months to 9 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 10 to 17 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from all participants enrolled in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 10 to 17 Years With a Serum Hemagglutination Inhibition Assay (HAI) Antibody Titer of 1:40 or Greater Against the Influenza H1N1 2009 Virus at Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from all participants enrolled in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. Each sample was tested at least twice according to standard operating procedures and the result of each replicate reported. A participant is counted if the geometric mean of the replicate values was 1:40 or greater.
  
  
• Number of Participants Age 6 to Less Than 36 Months With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 6 to Less Than 36 Months With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 21 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 36 Months to 9 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 36 Months to 9 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from participants enrolled after the first 30 in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 21 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 10 to 17 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 8-10 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 8-10 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was to be collected from the first 30 participants enrolled in each dose group in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 8-10 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 8-10 titer was an increase by 4-fold or more.
  
  
• Number of Participants Age 10 to 17 Years With 4-Fold or Greater HAI Antibody Titer Increases Against the Influenza H1N1 2009 Virus at Day 21 Following the Second Dose of H1N1 Vaccine [ Time Frame: Day 0 prior to first vaccination and Day 21 after the second vaccination ] [ Designated as safety issue: No ]
  Blood was collected from all participants enrolled in this age stratum for testing in the HAI assay with Influenza H1N1 2009 virus as the assay antigen. A participant met the threshold of a 4-fold rise in titer if the Day 0 titer was less than 1:10 (the assay's lowest level of detection) and the Day 21 titer was 1:40 or greater, or the Day 0 titer was greater than or equal to 1:10 and the Day 21 titer was an increase by 4-fold or more.
  
  

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 or equal to 36 months - 9 years age strata, the first 30 subjects enrolled in each dose level will have blood collected for antibody assays prior to vaccination on Day 0 - Visit 1, and then Day 8-10 - Visit 2 and Day 8-10 post second vaccination - Visit 4 only.