Determination of French influenza outbreaks periods between 1985 and 2011 through a web-based Delphi method

Abstract

Background: Assessing the accuracy of influenza epidemic periods determined by statistical models is important to improve the performance of algorithms used in real-time syndromic surveillance systems. This is a difficult problem to address in the absence of a reliable gold standard. The objective of this study is to establish an expert-based determination of the start and the end of influenza epidemics in France.

Methods: A three-round international web-based Delphi survey was proposed to 288 eligible influenza experts. Fifty-seven (20%) experts completed the three-rounds of the study. The experts were invited to indicate the starting and the ending week of influenza epidemics, on 32 time-series graphs of influenza seasons drawn using data from the French Sentinelles Network (Influenza-like illness incidence rates) and virological data from the WHO-FluNet. Twenty-six of 32 time-series graphs proposed corresponded to each of the French influenza seasons observed between 1985 and 2011. Six influenza seasons were proposed twice at each round to measure variation among expert responses.

Results: We obtained consensual results for 88% (23/26) of the epidemic periods. In two or three rounds (depending on the season) answers gathered around modes, and the internal control demonstrated a good reproducibility of the answers. Virological data did not appear to have a significant impact on the answers or the level of consensus, except for a season with a major mismatch between virological and incidence data timings.

Conclusions: Thanks to this international web-based Delphi survey, we obtained reproducible, stable and consensual results for the majority of the French influenza epidemic curves analysed. The detailed curves together with the estimates from the Delphi study could be a helpful tool for assessing the performance of statistical outbreak detection methods, in order to optimize them.

Figure 1

A typical question screen proposed to the experts during the completion of the questionnaire. The computer application provided epidemic curves to experts with an associated user-friendly slider (purple color, bottom left), the latter, with its two extreme handles, enabling experts to easily position the corresponding beginning and end of the outbreak. Just above the slider, a bar plot displaying the distribution of the answers collected at the previous round is provided as a feedback, with the own answer of the expert highlighted in a different (green) colour. The right part of the Internet page displays guiding information.

Figure 2

Organisation of the survey.

Figure 3

An epidemic curve standard display. Weekly national incidence rates of ILI data (blue dashed line) were obtained from the French Sentinelles Network. ILI was defined as: sudden fever > 39°C (102°F) with myalgia and respiratory signs. Weekly national proportions of confirmed influenza positive samples (virological data, red bar plot) data were downloaded from WHO-FluNet [17] (data provided by the French National Reference Center for flu, cell culture or PCR test, all influenza virus combined), for mainland France. The grey bar plot at the bottom of the graph corresponds to the number of virological samples analyzed per week.