Geographic and Temporal Trends in Influenzalike Illness, Japan, 1992–1999

摘要

From 1992 to 1999, we analyzed >2.5 million cases ofinfluenzalike illness (ILI). Nationwide influenza epidemicsgenerally lasted 3–4 months in winter. Kriging analysis,which illustrates geographic movement, showed that thestarting areas of peak ILI activity were mostly found inwestern Japan. Two spreading patterns, monotonous andmultitonous, were observed. Monotonous patterns in twoseasons featured peak ILI activity that covered all of Japanwithin 3 to 5 weeks in larger epidemics with new antigenicvariants of A/H3N2. Multitonous patterns, observed in theother five seasons, featured peak ILI activity within 12 to 15weeks in small epidemics without new variants. Applyingthe kriging method allowed better visualization and under-standing of spatiotemporal trends in seasonal ILI activity.This method will likely be an important tool for futureinfluenza surveillance in Japan