This study aimed to describe the geographical spread and temporal accumulation of cases during the 2009 influenza A (H1N1) epidemic in Japan. Using a method adopted by the National Epidemiological Surveillance of Infectious Diseases (NESID) to identify epidemics, we described the frequency and temporal change of the 2009 influenza A (H1N1) epidemic among designated public health center areas in Japan. We calculated the proportion of public health center areas with an epidemic by week from 1999 to 2010. The 2009 influenza A (H1N1) epidemic had several distinguishing features compared with previous seasonal influenza epidemics. For example, the 2009 epidemic started in autumn and peaked at week 49, almost 10 weeks earlier than that of previous seasonal influenza epidemics. Additionally, the 2009 epidemic had the highest total number of public health center areas reporting an epidemic within a season, but only the second highest proportion of areas with an epidemic in the peak week (78.5%). The duration of the epidemic was 17 weeks, which was longer than that of previous epidemics. The present study results shed light on the epidemiology of the 2009 influenza A (H1N1) pandemic and provide useful information to create strategies to prevent future pandemics.
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