Lessons from 40 years' surveillance of influenza in England and Wales

Abstract

The influenza virus continues to pose a significant threat to public health throughout the world. Current avian influenza outbreaks in humans have heightened the need for improved surveillance and planning. Despite recent advances in the development of vaccines and antiviral drugs, seasonal epidemics of influenza continue to contribute significantly to general practitioner workloads, emergency hospital admissions, and deaths. In this paper we review data produced by the Royal College of General Practitioners Weekly Returns Service, a sentinel general practice surveillance network that has been in operation for over 40 years in England and Wales. We show a gradually decreasing trend in the incidence of respiratory illness associated with influenza virus infection (influenza-like illness; ILI) over the 40 years and speculate that there are limits to how far an existing virus can drift and yet produce substantial new epidemics. The burden of disease caused by influenza presented to general practitioners varies considerably by age in each winter. In the pandemic winter of 1969/70 persons of working age were most severely affected; in the serious influenza epidemic of 1989/90 children were particularly affected; in the millennium winter (in which the NHS was severely stretched) ILI was almost confined to adults, especially the elderly. Serious confounders from infections due to respiratory syncytial virus are discussed, especially in relation to assessing influenza vaccine effectiveness. Increasing pressure on hospitals during epidemic periods are shown and are attributed to changing patterns of health-care delivery.

Fig. 1

Influenza-like illness (ILI) 1967/68 to 2006/07: mean weekly incidence rate per 100 000 population in 4-weekly periods. The influenza viruses (type, subtype and strain) are highlighted for winters of significant activity.

Fig. 2

The weekly incidence of Influenza-like illness (ILI) described by age for a selection of the more severe epidemics over the last 40 years. Age-specific rates are presented for the weeks surrounding the peak week of all-age incidence (week 0).

Fig. 3

Respiratory general practitioner (GP) episodes and respiratory hospital admissions in the elderly (⩾65 years), and all-age all-cause deaths from 1994/95 to 2003/04. Regression slopes have been superimposed on each series to highlight the conflicting trends.

Fig. 4

Average excess admissions () and excess bed days (–––) in influenza epidemic periods by age group over years 1989–2001.

Fig. 5

Clinical incidence of influenza-like illness (ILI) and acute bronchitis (⩾65 years), respiratory admissions (⩾65 years) and all-cause deaths (all-age) with influenza and respiratory syncytial virus (RSV) active periods superimposed for the years 1999/00 to 2003/04.