The burden of influenza and respiratory syncytial viruses in Japan, 2006–2014: A region- and age-specific excess mortality study

      Purpose: Influenza and respiratory syncytial virus (RSV) infections greatly contribute to morbidity and mortality in humans, but such epidemiological impact is yet to be assessed in the super-aged nations using up-to-date methods. Here we estimate pandemic and winter-seasonal excess mortality rates associated with influenza and RSV across prefectures in Japan from 2006 to 2014. We then assess the influence of socio-demographic, environmental and healthcare factors on our excess mortality estimates.
      Methods & Materials: We used non-parametric statistical methods that capture seasonal and long-term trends to estimate excess mortality rates from time series stratified according to 6 age groups and prefectures. We then employed statistical modelling to assess the associations between excess mortality and predictor variables.
      Results: Across the 8 seasons (2006–2014), seasonal influenza was on average associated with about 27,000 annual deaths in Japan, or about 20 per 100,000 population (and only about 8,100 annual deaths, about 6.0 deaths per 100,000 population in the 2009 influenza pandemic). RSV was annually associated with about 34,000 annual deaths (about 27 per 100,000 population). For both infectious diseases, excess mortality rates were highest among seniors, especially aged ≥ 80 years. Respiratory and circulatory disease mortality was found to be most associated with influenza, accounting for 89% of influenza-associated deaths and 85% of RSV-associated deaths, respectively. For comparison, the number of notified laboratory-confirmed deaths during the 2009 pandemic was about 40 times smaller than our estimate of the actual number of excess deaths associated with the 2009 A/H1N1 influenza pandemic in Japan. Variations in excess mortality estimates across prefectures for both influenza and RSV were mostly shaped by climatological factors.
      Conclusion: Strikingly, we report greater estimates of influenza-associated mortality for Japan compared to estimates published over a decade ago. Moreover, our results revealed comparable mortality burdens exerted by RSV and influenza in Japan. In particular, seniors are disproportionately affected, especially those aged ≥ 80 years, indicating that a rapidly aging population could exacerbate mortality associated with these respiratory diseases.