Population-based study on incidence, risk factors, clinical complications and drug utilisation associated with influenza in the United Kingdom.

This large population-based study using the UK-based General Practice Research Database was conducted to quantify influenza-related physician visits, clinical complications of and risk factors for influenza, and related drug use in all age groups from 1991 to 1996. A total of 141,293 subjects who had one or more diagnoses of influenza or influenza-like illness during the study period as well as the same number of age-, sex-, practice and calendar time-matched controls were identified. Adults aged 15-64 years had the highest influenza incidence rate. The risk of getting influenza was particularly increased for subjects with chronic respiratory conditions (asthma or chronic obstructive pulmonary disease, odds ratio 1.65, 95% confidence interval 1.60-1.70). Subjects with influenza were more likely to have a diagnosis of clinical complications than control subjects (relative risk 3.4, 95% confidence interval 3.3-3.6). The risk of developing clinical complications was highest for children and was elevated for subjects with certain underlying chronic conditions. In absolute terms, otherwise healthy adults (15-64 years) accounted for the greatest proportion of all influenza-related physician visits as well as clinical complications in this study population. Of the 141,293 subjects with influenza, 83,911 (59.4%) received drugs on prescription. The most frequently prescribed drugs were antibiotics (45.2%), followed by antipyretics/analgesics (22.5%). Influenza patients were approximately six times more likely to use drugs on prescription than controls. This analysis may lead to further analyses on the economic impact of influenza and the contribution of different population groups to that burden.