Read v2 codes for diagnoses of asthma, or recording that a previous diagnosis of asthma had resolved (resolved=1). For this project, the definition was designed to follow national guidance on indications for influenza vaccination, namely: Asthma that requires continuous or repeated use of inhaled or systemic steroids or with previous exacerbations requiring hospital admission. Individuals were defined as having asthma if there was a code for asthma at any time before the index date AND the latest asthma code prior to the index date was not an ""asthma resolved"" code AND EITHER (a) A prescription in the 12 months before start of flu season for an oral steroid OR (b) A prescription in the 12 months before start of flu seasonfor an inhaled steroid OR (c) An asthma emergency admision ever in the past

Influenza, Risk factors