COMMENTARY: Case definition for influenza surveillance

Case definition for influenza is the basis for at least three different activities. It is essential for individual diagnosis leading to decision for treatment of patients and eventually to decide prophylaxis of contacts. It is also a key element for surveillance of epidemics and of virus variation. It is finally very useful for all clinical studies of evaluation (vaccines, antivirals). The review of the different case definitions in different European countries presented in the paper by Aguilera et al. [1] in this issue describes a very confusing picture of the validity of comparisons between different countries. The same variations are also seen in other regions of the world [2]. Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens on the basis of symptoms alone. However, during laboratory-confirmed influenza outbreaks, the majority of persons seeking medical advice for upper respiratory tract infections are likely to be infected by influenza. Since other viruses, not only respiratory viruses, can induce clinical conditions very similar to influenza, laboratory confirmation is required between annual influenza epidemics. Clearly, it is difficult to establish common rules for clinical definitions of influenza in different countries with different health systems and organisations. It may surprise those who are not familiar with this elusive virus to read that ‘a number of international case definitions (of clinical influenza) exist, including those of WHO, and that none is uniformly applied’ and that ‘there is currently no international consensus on what should be the case definition for influenza’.