Age-dependent estimates of the epidemiological impact of pandemic influenza (H1N1-2009) in Japan

Comput Math Methods Med. 2013;2013:637064. doi: 10.1155/2013/637064. Epub 2013 Feb 21.


The total number of influenza cases with medical attendance has been estimated from sentinel surveillance data in Japan under a random sampling assumption of sentinel medical institutions among the total medical institutions. The 2009 pandemic offered a research opportunity to validate the sentinel-based estimation method using the estimated proportion of infections measured by the population-wide seroepidemiological survey employing hemagglutinin inhibition (HI) assay. For the entire population, we estimated the age-standardized proportion of infections at 28.5% and 23.5% using cut-off values of HI titer at 1 : 20 and 1 : 40, respectively. Investigating the age profiles, we show that the estimated influenza-like illness (ILI) cases with medical attendance exceeded the estimated infections among those aged from 0 to 19 years, indicating an overestimation of the magnitude by sentinel-based estimation method. The ratio of estimated cases to estimated infections decreased as a function of age. Examining the geographic distributions, no positive correlation was identified between the estimated cases and infections. Our findings indicate a serious technical limitation of the so-called multiplier method in appropriately quantifying the risk of influenza due to limited specificity of ILI and reporting bias. A seroepidemiological study should be planned in advance of a pandemic.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Age Distribution
  • Child
  • Child, Preschool
  • Data Interpretation, Statistical
  • Geography
  • Hemagglutinin Glycoproteins, Influenza Virus / chemistry
  • Humans
  • Incidence
  • Infant
  • Influenza A Virus, H1N1 Subtype*
  • Influenza, Human / epidemiology*
  • Japan
  • Middle Aged
  • Models, Statistical
  • Pandemics
  • Risk
  • Young Adult


  • H1N1 virus hemagglutinin
  • Hemagglutinin Glycoproteins, Influenza Virus