Spanish Flu, Asian Flu, Hong Kong Flu, and Seasonal Influenza in Japan under Social and Demographic Influence: Review and Analysis Using the Two-Population Model

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When cumulative numbers of patients (X) and deaths (Y) associated with an influenza epidemic are plotted using the log-log scale, the plots fall on an ascending straight line generally expressed as logY = k(logX − logN0). For the 2009 H1N1 influenza pandemic, the slope k was ～0.6 for Mexico and ～2 for other countries. The two-population model was proposed to explain this phenomenon (Yoshikura H. Jpn J Infect Dis. 2012;65:279-88; Yoshikura H. Jpn J Infect Dis. 2009;62:411-2; and Yoshikura H. Jpn J Infect Dis. 2009;62:482-4). The current article reviews and analyzes previous influenza epidemics in Japan to examine whether the two-population model is applicable to them. The slope k was found to be ～2 for the Spanish flu during 1918–1920 and the Asian flu during 1957–1958, and ～1 for the Hong Kong flu and seasonal influenza prior to 1960–1961; however, k was ～0.6 for seasonal influenza after 1960–1961. This transition of the slope k of seasonal influenza plots from ～1 to ～0.6 corresponded to the shift in influenza mortality toward the older age groups and a drastic reduction in infant mortality rates due to improvements in the standard of living during the 1950s and 1960s. All the above observations could be well explained by reconstitution of the influenza epidemic based on the two-population model.

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