An Annual Epidemic of Influenza in Japan Analyzed with Network Analysis

Background: In this decade, several countries have suffered from infection of humans with highly pathogenic H5N1 influenza, provoking the threat of a worldwide pandemic. Information about a spread pattern of infectious diseases such as influenza of the currently ordinary types H3N2 and H1N1 is thought to be useful for taking preventative measures against this misgiving pandemic. This study aimed to analyze annual epidemics of the ordinary influenza in Japan with network analysis and to explore spread patterns. Methods: The analyzed data was the cases of influenzalike-illness during these 20 years. We constructed the vectors of the increasing rates of the reported cases for each prefecture at each week, and calculated the matrices of the space-time correlation which was defined for all pairs of prefectures. As each correlation was treated as each tie, a valued-network was depicted, and was analyzed with network analysis. All prefectures were also classified with cluster analysis. Results: In 2007, the ties between prefectures were strong in the south-east region of Japan, and the strongest tie was that between Miyazaki and Kagoshima. Relatively strong ties could be that between geographically neighboring prefectures. On the other hand, the ties were relatively weak for a prefecture isolated from the others by seas (Hokkaido or Okinawa). The trend seemed almost similar during analyzed years. Conclusion: An epidemic could be spread within geographically neighboring prefectures connected with a strong tie. These prefectures should be targeted to organize preventative measures of pandemic of H5N1 intensively.