title: A perspective on multiple waves of influenza pandemics.
abstract: BACKGROUND
  A striking characteristic of the past four influenza pandemic outbreaks in the United States has been the multiple waves of infections. However, the mechanisms responsible for the multiple waves of influenza or other acute infectious diseases are uncertain. Understanding these mechanisms could provide knowledge for health authorities to develop and implement prevention and control strategies.
  
  
  METHODS
  We exhibit five distinct mechanisms, each of which can generate two waves of infections for an acute infectious disease. The first two mechanisms capture changes in virus transmissibility and behavioral changes. The third mechanism involves population heterogeneity (e.g., demography, geography), where each wave spreads through one sub-population. The fourth mechanism is virus mutation which causes delayed susceptibility of individuals. The fifth mechanism is waning immunity. Each mechanism is incorporated into separate mathematical models, and outbreaks are then simulated. We use the models to examine the effects of the initial number of infected individuals (e.g., border control at the beginning of the outbreak) and the timing of and amount of available vaccinations.
  
  
  RESULTS
  Four models, individually or in any combination, reproduce the two waves of the 2009 H1N1 pandemic in the United States, both qualitatively and quantitatively. One model reproduces the two waves only qualitatively. All models indicate that significantly reducing or delaying the initial numbers of infected individuals would have little impact on the attack rate. Instead, this reduction or delay results in a single wave as opposed to two waves. Furthermore, four of these models also indicate that a vaccination program started earlier than October 2009 (when the H1N1 vaccine was initially distributed) could have eliminated the second wave of infection, while more vaccine available starting in October would not have eliminated the second wave.
authors: Department of Mathematics, Marshall University, Huntington, West Virginia, United States of America.
authors: Anna
authors: A
authors: Mummert
authors: Howard
authors: H
authors: Weiss
authors: Li-Ping
authors: LP
authors: Long
authors: José M
authors: JM
authors: Amigó
authors: Xiu-Feng
authors: XF
authors: Wan
mesh_terms: D004198:Disease Susceptibility / Q000821:virology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000502:physiology; D007252:Influenza Vaccines / Q000008:administration & dosage; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control / Q000635:transmission*; D008954:Models, Biological; D015233:Models, Statistical; D009154:Mutation; D058873:Pandemics / Q000706:statistics & numerical data*; D013997:Time Factors; D014481:United States / Q000453:epidemiology; D014611:Vaccination
chemical_list: D007252:Influenza Vaccines
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