The contribution of influenza to combined acute respiratory infections, hospital admissions, and deaths in winter.

The impact of influenza is assessed by comparing events during epidemics with those expected outside epidemic periods (defined from incidence data collected by the Weekly Returns Service of the Royal College of General Practitioners from 1989 to 1998 for influenza-like illness, acute otitis media, acute bronchitis, and all respiratory infections combined in the community and virus isolate data). Estimates of the consulting populations for each condition in England and Wales were derived by extrapolating the difference between observed and baseline incidence rates to the total population. Similar methods were applied to data on hospital admissions for cardiac and respiratory diseases and to deaths. Each year an average of 422,000 extra people consulted and were diagnosed with influenza-like illness during the epidemic period; among 1.1 million extra people who presented with acute respiratory infections. There were 3028 excess respiratory admissions (England only) in the age group 65 to 74 years and 6049 who were aged over 75 years, but no excess cardiac admissions. An average of 12,554 deaths occurred in England and Wales during influenza epidemic periods each year. Age specific national data are needed to interpret the economic impact of an illness in relation to the setting for health care delivery.