Incidence data by age of new episodes of influenza-like illness reported by sentinel general
practice networks in England and Wales and in The Netherlands over a 10-year period were
examined to provide estimates of the consulting population during influenza epidemic periods.
Baseline levels of recording in each age group were calculated from weeks in which influenza
viruses were not circulating and the excess over baseline calculated to provide the 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 65 years and over. In the
intervening age groups, population estimates were more consistent regardless of the virus type.
Both networks reported simultaneous peaking of incidence rates in all of the age groups. There
were substantial increases in the number of persons reporting other respiratory illnesses during
Population estimates of the consulting population provide the only secure basis for which
health services resource utilization during influenza epidemics can be estimated.