Influenza case definitions need to be fit for purpose

The recent paper by Jiang et al. [1] provides an in-depth analysis of the performance of different case definitions for detecting influenza A(H1N1) pdm09 among adults in the community, including those who have not presented for medical attention.The authors concluded that the revised World Health Organization (WHO) influenza-like illness (ILI) definition was an improvement on alternative definitions used by the United States Centers for Disease Control and Prevention (CDC) and the European Centre for Disease Prevention and Control (ECDC), based on its higher positive predictive value (PPV) and higher likelihood ratio for a positive test. However, these are not necessarily the most important measures of success for all ILI case definitions, and different definitions will be required for different purposes. Identifying influenza for surveillance, for case finding during activities to prevent spread during an epi-demic, and for case finding for participation in research studies of treatment or prevention of transmission will require different case definitions.