Spatiotemporal analysis of influenza epidemics in Japan

Abstract

An influenza epidemic is a complicated phenomenon influenced by numerous social and biological factors, including geography, climate, population, transport network, and biological statuses of humans and viruses, among others. To investigate the strength of these influences, we evaluated data from influenza epidemics that occurred in Japan between April 1999 and December 2014 using wavelet analysis. We calculated wavelet transform and phase difference, which was defined as the phase difference in a prefecture other than Tokyo. The time-averaged phase differences revealed the following: (1) the epidemics were earlier and more strongly synchronized in 7 prefectures in the Kanto region, which includes Tokyo, and in 7 prefectures in the Kinki region, which includes Osaka; (2) except for these urban regions, the epidemics propagated from western to eastern prefectures, and finally to northern prefectures; (3) epidemic jumps occurred in several prefectures (e.g., Miyagi Prefecture); and (4) epidemics occasionally occurred at different times in two adjacent prefectures (e.g., Yamanashi and Tokyo). We then attempted to qualitatively deduce the causes for these observations. This study is expected to be important for integrating knowledge to derive trends in epidemics, both nationally and internationally.