A confirmed case is defined as an individual with a laboratory confirmed influenza infection admitted to ICU/HDU. Prior to the 2009 influenza pandemic there was a recognised gap in surveillance of severe respiratory infection in the UK, in particular with regards to hospitalised cases of influenza.

A pilot surveillance scheme for severe influenza, the UK Severe Influenza Surveillance System (USISS), was initiated during the 2010/11 post-pandemic influenza season in order to monitor and estimate the impact of seasonal influenza on the population and to describe the epidemiology of severe disease.

The system was subsequently rolled out in the 2011/12 influenza season and has continued to operate since and is mandated for all NHS trusts.

The objectives of the collection are to:

1. Monitor and estimate the impact of influenza (both seasonal and pandemic) on the population
2. Describe the epidemiology of severe influenza (ICU admissions and deaths) in time, place and person
3. Be able to rapidly identify and describe the epidemiological features of a novel influenza virus
4. To monitor the impact of the introduction of the childhood influenza vaccination programme.