Temporal and geographical variation in epidemics determined from the results of an infectious disease surveillance system in Japan description of epidemic patterns by data-based criteria for epidemic periods

OBJECTIVE: After developing criteria for epidemic periods for 16 infectious diseases, we investigated temporal (annual and monthly) and geographical (regional) variation in epidemics. METHODS: Data from an infectious disease surveillance system for the years 1993 to 1997 were used for the analysis. The weekly number of patients per monitoring station was calculated from the data and used in developing criteria for an epidemic period. Based on these latter, we calculated the average number of epidemics occurring at each public health center in a year, an average length of the epidemic period, and the average number of patients per monitoring station reported during an epidemic period. These figures were used to explore temporal (annual, monthly) and geographical (regional) variation. RESULTS: With most diseases, the average number of epidemics occurring at a public health center was 0.3-0.5 per year and the average length of an epidemic period was 6-12 weeks. The average number of patients per monitoring station reported during an epidemic period differed according to the diseases. These indices showed that there were few annual differences, but great monthly differences, in most diseases. The average number of epidemics occurring in a public health center showed regional differences with some diseases. CONCLUSION: Temporal and geographical variation in epidemics for 16 infectious diseases was established by developing criteria for an epidemic period.