This study assesses the seasonal of influenza vaccine effectiveness in adults hospitalised with laboratory-confirmed influenza through a network of hospitals in France

Influenza vaccination strategies aim at protecting high-risk population from severe outcomes. Estimating the effectiveness of seasonal vaccines against influenza related hospitalisation is important to guide these strategies. The objective is to measure the population effectiveness of influenza vaccine in patients hospitalized with virologically confirmed influenza laboratory during the influenza season 2012/2013 in a French hospitals network. This is a study "case-control" involving a nasopharyngeal sample for all hospitalised patients presenting an influenza-like illness within seven days. Cases will be patients RT-PCR positive for influenza. Controls will be patients negative for any influenza virus.

Inclusion Criteria:

• Age ≥ 18 year
• Affiliated with social security health insurance
• Written informed consent
• Patients hospitalized for 24 hours and presenting an influenza-like illness in the last 7 days before the hospitalization

Exclusion Criteria:

• Against indication for influenza vaccination(Hypersensitivity to the active substances, to any of the excipients and to trace eg eggs, including ovalbumin, chicken protein)
• Patients institutionalized without regular community interaction
• Patient already hospitalized for respiratory infection during the 2012-2013 influenza season
• Flu already virologically documented in the current influenza season 2012-2013 (RT-PCR, multiple RT-PCR and / or culture.)