Population estimates of persons presenting to general practitioners with influenza-like illness, 1987-96: a study of the demography of influenza-likeillness in sentinel practice networks in England and Wales, and in The Netherlands

Incidence data by age of new episodes of influenza-like illness reported by sentinel general practice networks in England and Wales and in The Netherl ands over a 10-year period were examined to provide estimates of the consul ting population during influenza epidemic periods. Baseline levels of recor ding in each age group were calculated from weeks in which influenza viruse s were not circulating and the excess over baseline calculated to provide t he population estimates during influenza epidemics. Influenza A/H3N2 epidemics were associated with higher population estimates for consultations than influenza B, especially in the age groups 0-4 and 6 5 years and over. In the intervening age groups, population estimates were more consistent regardless of the virus type. Both networks reported simult aneous peaking of incidence rates in all of the age groups. There were subs tantial increases in the number of persons reporting other respiratory illn esses during influenza epidemics. Population estimates of the consulting population provide the only secure b asis for which health services resource utilization during influenza epidem ics can be estimated.