title: Transmissibility and temporal changes of 2009 pH1N1 pandemic during summer and fall/winter waves.
abstract: BACKGROUND
  In order to compare the transmissibility of the 2009 pH1N1 pandemic during successive waves of infections in summer and fall/winter in the Northern Hemisphere, and to assess the temporal changes during the course of the outbreak in relation to the intervention measures implemented, we analyze the epidemiological patterns of the epidemic in Taiwan during July 2009-March 2010.
  
  
  METHODS
  We utilize the multi-phase Richards model to fit the weekly cumulative pH1N1 epidemiological data (numbers of confirmed cases and hospitalizations) as well as the daily number of classes suspended under a unique "325" partial school closing policy in Taiwan, in order to pinpoint the turning points of the summer and fall/winter waves, and to estimate the reproduction numbers R for each wave.
  
  
  RESULTS
  Our analysis indicates that the summer wave had slowed down by early September when schools reopened for fall. However, a second fall/winter wave began in late September, approximately 4 weeks after the school reopened, peaking at about 2-3 weeks after the start of the mass immunization campaign in November. R is estimated to be in the range of 1.04-1.27 for the first wave, and between 1.01-1.05 for the second wave.
  
  
  CONCLUSIONS
  Transmissibility of the summer wave in Taiwan during July-early September, as measured by R, was lower than that of the earlier spring outbreak in North America and Europe, as well as that of the winter outbreak in Southern Hemisphere. Furthermore, transmissibility during fall/winter in Taiwan was noticeably lower than that of the summer, which is attributable to population-level immunity acquired from the earlier summer wave and also to the intervention measures that were implemented prior to and during the fall/winter wave.
authors.affiliation: Department of Public Health, China Medical University, Taichung 40402, Taiwan. hsieh@mail.cmu.edu.tw
authors.forename: Ying-Hen
authors.initials: YH
authors.lastname: Hsieh
authors.forename: Kuang-Fu
authors.initials: KF
authors.lastname: Cheng
authors.forename: Trong-Neng
authors.initials: TN
authors.lastname: Wu
authors.forename: Tsai-Chung
authors.initials: TC
authors.lastname: Li
authors.forename: Chiu-Ying
authors.initials: CY
authors.lastname: Chen
authors.forename: Jin-Hua
authors.initials: JH
authors.lastname: Chen
authors.forename: Mei-Hui
authors.initials: MH
authors.lastname: Lin
mesh_terms: D050936:Basic Reproduction Number / Q000706:statistics & numerical data; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D058873:Pandemics; D012621:Seasons; D013624:Taiwan / Q000453:epidemiology