Influenza virus infections are a major cause of morbidity and mortality. The limited existing knowledge about the impact of influenza in immunocompromised patients suggests that they are at increased risk of influenza virus acquisition, of developing complications and of prolonged illness and viral shedding. However, some other data about the effect of antiviral agents on the infection course, and risk of resistance in immunocompromised children are lacking. The emergence of the pandemic H1N1 swine-origin influenza A virus has generated an additional need to study the epidemiology, clinical course and outcome of influenza infections in immunocompromised children. This study proposed to conduct a prospective observational clinical study to answer these questions.

This study aims to do the following;

• To evaluate the frequency of influenza virus infections due to various subtypes in immunocompromised children and young adults at SJCRH
• To describe and compare the clinical course and outcome of pandemic H1N1 influenza infection with that due to other influenza virus subtypes in immunocompromised children and young adults.
• To evaluate the duration of influenza virus shedding in immunocompromised children and young adults.
• To evaluate the immunologic response to natural infection with various subtypes of influenza virus including the pandemic H1N1 influenza A virus.
• To evaluate the viral resistance to antiviral agents in relation to antiviral therapy.
• To compare accuracy of rapid methods for influenza A subtyping and for determination of antiviral resistance.

Inclusion Criteria:

• Age <= 21 years of age at the time of entry into the study.
• Body weight of 13 kg or greater
• An immunosuppressed state due to cancer, sickle cell disease, HIV, or receipt of stem cell transplant
• Presentation with acute respiratory illness defined as recent onset of rhinorrhea, nasal and sinus congestion, pharyngitis, coryza, sinusitis, otitis media, dyspnea or shortness of breath, cough and/or a new radiographic pulmonary infiltrates.
• Proven influenza virus infection by virological testing of respiratory specimens using Polymerase Chain reaction (PCR) assay, direct antigen detection assay, or viral culture.

Exclusion Criteria:

• Inability or unwillingness of research participant or legal guardian to give written informed consent.

This is a prospective surveillance study that evaluates research participants who are ≤ 21 years of age and diagnosed at SJCRH with acute respiratory illness due to influenza virus confirmed by antigen detection, PCR or viral culture of clinical respiratory specimens.