INFLUENZA-A AND INFLUENZA-B EPIDEMIC CRITERIA-BASED ON TIME-SERIES ANALYSIS OF HEALTH-SERVICES SURVEILLANCE DATA

Many countries now have epidemiological surveillance systems using hea lth services-based indicators that allow detection of influenza epidem ics. However, there is no accepted criterion for defining an influenza epidemic. An epidemic criterion has been developed, based on a time-s eries analysis of health services-based indicators collected on a week ly basis by a surveillance network implemented in the Paris region sin ce 1984: the Groupe Regional d'Observation de la Grippe (GROG). For ea ch new season, an epidemic threshold is independently defined for each health services-based indicator as the upper limit of the one-sided c onfidence interval of the expected value calculated from the weekly di fferences between the observed number of events and those predicted by a SARIMA model fitted on the non-epidemic.data of previous seasons. E pidemic criteria for influenza A and B are then defined from the combi nation of both viral indicators and epidemic thresholds of individual health services-based indicators. Among health indicators, sick-leave data collected from GP's or the Health Insurance system, emergency hom e medical visits, and influenza-like-illness reported by GP's are the most sensitive indicators for the early recognition of epidemics. The exceeding of the above mentioned thresholds combined with virological data allows the specific detection of influenza A or B epidemics. This time-series method of analysing surveillance data provides early and reliable recognition of these epidemics.