A new influenza surveillance system in France: the Ile-de-France "GROG". 2. Validity of indicators (1984-1989).

The evaluation of the validity of the GROG's indicators is based on the first five years of activity, four of which being epidemic ones. A virological criterion for identifying epidemics is used as the gold standard. For each non-specific indicator, the ratio of epidemic weeks to non-epidemic weeks is outlined. Therefore we can define four epidemic situations in relation with the strains circulating and the non-specific indicators. Influenza A epidemics, which are known as a mortality problem can also be described according to its impact upon morbidity. As a result, it is possible to set up a sensitive and specific surveillance system for early detection of influenza epidemics. This objective is achieved by combining both the weekly collection of virological data as well as the collection of other available indicators either in routine practice or as provided by a network of sentinel practitioners. Such detection can pave the way for an improvement in preventive measures against the harmful effects on health of influenza epidemics.