Spatio-Temporal Characteristics of Influenza Burden and Its Influence Factors in Japan in the Past Three Decades: An Influenza Disease Burden Data-Based Modeling Study

Introduction: Influenza has still posed a great threat to humans. The knowledge of the systematic disease burden of influenza in Japan was limited. The study was aimed to investigate Spatio-temporal characteristics of the influenza burden and its influence factors in the past three decades.

Methods: Data on annual death, years lived with disability (YLDs), years of life lost (YLLs) and disability adjusted life year (DALYs) of influenza from 1990 to 2019 in Japan were available from the Global Health Data Exchange (GHDx), and data on annual social household available from e-Stat in Japan. A joinpoint regression model was used to assess the trends of influenza from 1990 to 2019, a discrete Poisson model to analyze the spatial and temporal cluster of influenza, and a generalized linear model to assess the association of death and DALY of influenza with social household factors.

Results: From 1990 to 2019, the mortality rate increased from 9.95 per 100000 to 19.49 per 100000 in Japan, with AAPC of 2.2% (95% CI: 1.5, 3.0, P<0.05). The DALYs rate increased from 153.86 per 100000 to 209.22 per 100000, with AAPC of 1.0% (95% CI: 0.1, 1.9, P<0.05). The mortality rate ranged from 1.98 per 100000 (Chiba) to 16.9 per 100000 (Kochi) in 1990, and from 5.10 per 100000 (Chiba) to 35.74 per 100000 (Akita) in 2019. The population aged 60+ had the highest mortality rates from 53.79 per 100000 in 1990 to 55.74 per 100000 in 2019 (AAPC: 0.0%, 95% CI: -0.5, 0.6, P=0.944) and DALYs rates from 713.43 per 100000 to 565.22 per 100000 (AAPC: -0.9%, 95% CI: -1.5, -0.3, P<0.05). YLLs and DALYs rates among the population aged 1-4 were also high from 1990 to 2019, ranked after that among populations aged 60+. The mortality rate had two stages of spatio-temporal aggregation across Japan, northern Japan with the period of 2005-2019 (RR = 1.36, P < 0.001) and southern Japan with the same period in the northern area (RR = 1.36, P < 0.001). The generalized linear model (GLM) indicated that year was positively correlated with the mortality rate of influenza (β = 0.18, p<0.01); while the ratio of households ordered via the internet and population were negatively correlated with the mortality rate of influenza (β = -4.41, p<0.05 and β =-0.17, p<0.01, respectively).

Conclusions: The disease burden of influenza in Japan increased in the past three decades, especially among the population aged 60+ years, followed by the population aged 1-4 years. It had two stages of spatio-temporal aggregation across Japan. Lifestyle of households ordered via the internet contributed to the low mortality rate of influenza.