This surveillance study will include all consenting adults hospitalized with influenza (cases) at the SOS network of hospitals as well as influenza negative patients(controls), to assess vaccine effectiveness in each group. Both groups will be followed throughout their hospitalization and for 30 days post discharge to monitor severity of illness and outcomes following their illness.

The Canadian Immunization Research Network(CIRN) is a collaborative research network which brings together over 100 investigators in over 40 institutions to evaluate all factors relevant to the introduction of a pandemic influenza vaccine in Canada. The Serious Outcomes Surveillance (SOS) Network within CIRN was established in 2009 to plan for evaluation of influenza vaccine safety and effectiveness. Vaccine effectiveness studies are critical in assessing vaccine effectiveness under real world conditions and in assessing vaccine effectiveness in the prevention of severe outcomes. Establishing the methodology and infrastructure for real-time field assessment of vaccine effectiveness is crucial to inform policy recommendations for the optimal use of resources, including vaccines.

This study will include all consenting adult patients hospitalized at one of the SOS Network hospitals during influenza season who test positive for influenza (cases). In addition two influenza negative controls will be matched to each case to measure differences in vaccine effectiveness. Specific study aims are:

1. To determine the effectiveness of trivalent influenza vaccination (TIV in general, and GSK TIV in particular) in preventing influenza-associated hospitalization in adults ≥ 65 years,
2. To determine the effectiveness of influenza vaccination in preventing influenza-associated death in adults ≥ 65 years
3. To determine the effectiveness of influenza vaccination in preventing influenza-associated hospitalization and death in adults < 65 years
4. To characterize the burden of disease, clinical outcomes, and resource utilization associated with influenza A and influenza B lineages
5. To examine clinical and immunologic factors impacting on severity of disease and influenza vaccine effectiveness in adults

Inclusion Criteria:

• Adult patients ≥ 16 years of age admitted to participating SOS Network hospitals with the following admitting diagnoses will be eligible for screening:
• pneumonia
• acute exacerbation of chronic obstructive pulmonary disease (AECOPD) or asthma
• unexplained sepsis
• Any other respiratory infection or diagnosis Or any respiratory or influenza-like symptom )(eg dyspnea, cough, sore throat, myalgia, arthralgia, fever, delirium/altered level of consciousness, CHF)

Exclusion Criteria:

• Patients whose reason for admission was clearly unrelated to the presence of influenza (for example, patients admitted due to trauma, elective surgery, or patients who have an alternative diagnosis that is clearly not respiratory,, e.g. cellulitis, intra-abdominal process, or gastrointestinal bleeding) Unless being enrolled as a nosocomial influenza case
• Patients whose onset of symptoms was prior to or within 72 hours of hospital admission but who were not tested for influenza within 7 days of hospital admission. These should be captured on the screening form as screen failures.
• No children in care will be enrolled in the study

This observational study is open to adults admitted to hospital with test postitive influenza or with an influenza like illness who test negative.