This study will evaluate the immunogenicity and safety of inactivated H1N1 swine-origin monovalent influenza vaccine in immunocompromised children and young adults.

This is a prospective observational study. This study will not dictate vaccine administration, nor the vaccine product to be used. It will collect data in St Jude patients who will receive the vaccine as part of their clinical care per institutional guidelines. Research participants will receive two doses of inactivated H1N1 swine-origin monovalent influenza vaccine administered 28 days apart as part of clinical care. Each dose contains 15 mcg of hemagglutinin antigen (HA).Follow-up visits will be scheduled 28 days after each dose and at day 208+/- 14 days for blood work. The day of administration of the first vaccine dose is counted as day 0. At each of these visits, blood will be collected to assess for immunogenicity. Research participants will be given a diary card to record symptoms for 28 days . They will be asked to bring this diary back at the next visit. Other vaccine-related adverse events will also be collected by retrospective chart review.

Children and young adults between the ages of 6 months and 21 years and 13 kg or greater in body weight who have an underlying diagnosis of cancer, HIV, sickle cell disease or receipt of a stem cell transplant more than a year prior to study entry and who will receive inactivated H1N1 swine-origin monovalent influenza vaccine in the winter/fall of 2009-2010 as part of their routine clinical care.

Inclusion Criteria:

• Age 6 months through 21 years of age at the time of entry into the study;
• Body weight of 13 kg or greater at the time of entry into the study;
• Participant is immunocompromised due to cancer, HIV, or sickle cell disease or receipt of a stem cell transplant;
• Participant or participant's parent/legal guardian available by telephone during the course of the study;
• Written informed consent (and assent, if applicable) obtained;
• Potential research participant received or will receive H1N1 swine-origin monovalent influenza vaccine within 24 hours of study entry.

Exclusion Criteria:

• Proven history, by RT-PCR, of H1N1 swine-origin influenza infection prior to study entry;
• Receipt of a stem cell transplant within one year prior to study entry;
• Participation in P1088 study.

Children and young adults between the ages of 6 months and 21 years and 13 kg or greater in body weight who have an underlying diagnosis of cancer, HIV, sickle cell disease or receipt of a stem cell transplant more than a year prior to study entry and who will receive inactivated H1N1 swine-origin monovalent influenza vaccine in the winter/fall of 2009-2010 as part of their routine clinical care. Stem cell transplant recipients in the first year following their transplant are excluded.