Detection of epidemics in their early stage through infectious disease surveillance

Background. Surveillance of infectious diseases is done in many countries. The aims of such surveillance include the detection of epidemics. In the present study, the possibility of detecting an epidemic in its early stage using a simple method was evaluated for 16 infectious diseases. Methods. We used as an index the number of cases per week per sentinel medical institution in the area covered by a health centre in infectious disease surveillance in Japan in 1993-1997. Periods of epidemics in health centre areas were determined according to the reported indices. The simple method used for detecting the early stage of an epidemic is that if the index exceeds a critical value, then an epidemic will begin in the following 4 weeks. The sensitivity, specificity and positive predictive value for this epidemic warning were evaluated for given critical values. Results. When the specificity of the epidemic warning was more than 95%, the sensitivity was more than 60% in ten diseases, and more than 80% in four diseases (influenza-like illness, rubella, hand-foot-and-mouth disease, and herpangina). The positive predictive value was between 15.6% and 31.4% in these ten diseases. Conclusion. The early stage of epidemics of some infectious diseases might be detectable using this simple method.