From 1992 to 1999, we analyzed > 2.5 million cases of influenzalike illness (ILI). Nationwide influenza epidemics generally lasted 3-4 months in winter. Kriging analysis, which illustrates geographic movement, showed that the starting areas of peak ILI activity were mostly found in western Japan. Two spreading patterns, monotonous and multitonous, were observed. Monotonous patterns in two seasons featured peak ILI activity that covered all of Japan within 3 to 5 weeks in larger ...
<75,000 would have one sentinel, a population 75,000-125,000 would have two, and populations><10 years of age, and in 1998 to 1999, the age distribution was mostly>