Abstract
BackgroundDuring the 2009 influenza pandemic period, routine surveillance of influenza-like-illness (ILI) was conducted in The Netherlands by a network of sentinel general practitioners (GPs). In addition during the pandemic period, four other ILI/influenza surveillance systems existed. For pandemic preparedness, we evaluated the performance of the sentinel system and the others to assess which of the four could be useful additions in the future. We also assessed whether performance of the five systems was influenced by media reports during the pandemic period.MethodsThe trends in ILI consultation rates reported by sentinel GPs from 20 April 2009 through 3 January 2010 were compared with trends in data from the other systems: ILI cases self-reported through the web-based Great Influenza Survey (GIS); influenza-related web searches through Google Flu Trends (GFT); patients admitted to hospital with laboratory-confirmed pandemic influenza, and detections of influenza virus by laboratories. In addition, correlations were determined between ILI consultation rates of the sentinel GPs and data from the four other systems. We also compared the trends of the five surveillance systems with trends in pandemic-related newspaper and television coverage and determined correlation coefficients with and without time lags.ResultsThe four other systems showed similar trends and had strong correlations with the ILI consultation rates reported by sentinel GPs. The number of influenza virus detections was the only system to register a summer peak. Increases in the number of newspaper articles and television broadcasts did not precede increases in activity among the five surveillance systems.ConclusionsThe sentinel general practice network should remain the basis of influenza surveillance, as it integrates epidemiological and virological information and was able to maintain stability and continuity under pandemic pressure. Hospital and virological data are important during a pandemic, tracking the severity, molecular and phenotypic characterization of the viruses and confirming whether ILI incidence is truly related to influenza virus infections. GIS showed that web-based, self-reported ILI can be a useful addition, especially if virological self-sampling is added and an epidemic threshold could be determined. GFT showed negligible added value.
Highlights
During the 2009 influenza pandemic period, routine surveillance of influenza-like-illness (ILI) was conducted in The Netherlands by a network of sentinel general practitioners (GPs)
Our aim was to assess the performance of the routine influenza surveillance system (ILI consultation rates reported by sentinel GPs) and whether the other available surveillance systems would be useful additions to the sentinel system
Trends in ILI/influenza The ILI consultation rates reported by the sentinel GPs (Figure 1A) and the incidence of hospital admissions due to influenza A(H1N1)pdm09 (Figure 1B) both peaked in the week of 9 November 2009
Summary
During the 2009 influenza pandemic period, routine surveillance of influenza-like-illness (ILI) was conducted in The Netherlands by a network of sentinel general practitioners (GPs). In addition during the pandemic period, four other ILI/influenza surveillance systems existed. The Dutch Continuous Morbidity Registration sentinel general practice network was established in 1970 for surveillance of influenza-like illness (ILI). During the 2009 pandemic in The Netherlands, four other surveillance systems were used These included an internet-based monitor of self-reported ILI symptoms in the Dutch and Belgian general population: the Great Influenza Survey (GIS) [7,8]. During the pandemic period, all hospitals were required to report patients admitted due to laboratory-confirmed influenza A(H1N1)pdm virus infection [10,11]
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