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Review Article
Influenza/respiratory viruses
INTRODUCTION
In common with other northern hemisphere countries, circulation of influenza is seasonal in the UK, with peak activity typically seen between November and March each year. As seasonal influenza epidemics result in substantial mortality and morbidity in the population and cause sickness absence from work and schools, rapid detection of the onset of influenza and monitoring of the intensity of activity each season are key objectives of influenza surveillance to ensure timely mobilization of health service resources and allocation of appropriate control and prevention interventions, in particular triggering use of antivirals in the community.
Several surveillance schemes are in place for indicating the start and assessing the intensity of influenza activity (defined for the rest of this paper as the magnitude of monitored rates) in the population. These include General Practitioner (GP) and other syndromic surveillance schemes (monitoring in near real-time data on patients presenting to healthcare services with signs and symptoms suggestive, or characteristic of influenza [1]), laboratory reporting, hospitalizations and mortality surveillance [2]. Within the UK, there are several sentinel GP surveillance schemes monitoring influenza-like illness (ILI) consultation rates in primary care. Each geographically distinct scheme has a distinct pre-assigned threshold which, when breached, denotes the start of significant influenza circulation in the community. An important clinical consequence of the threshold is its use, along with other influenza surveillance indicators, to inform the decision to trigger the prescription of antiviral use in the community based on National Institute for Health and Clinical Excellence (NICE) guidelines [3, 4]. For each scheme, threshold assignment has historically been based on both statistical assessment and epidemiological judgement. Threshold values are occasionally reviewed in response to observed changes in the surveillance scheme, the data received and other factors such as changes in trends in healthcare-seeking behaviour [5, 6]. Despite these revisions, the current thresholds used in the various UK schemes appear to be inconsistent with the lower rates of activity reported in recent seasons. This inconsistency can result in the threshold failing to provide an early warning of the start of influenza activity and consequently reducing its relevance to public health professionals.
While factors such as ILI definition adherence and consistency in recording can differ between sentinel...