title: Mechanisms of replacement of circulating viruses by seasonal and pandemic influenza A viruses.
abstract: BACKGROUND
  Seasonal influenza causes annual epidemics by the accumulation of antigenic changes. Pandemic influenza occurs through a major antigenic change of the influenza A virus, which can originate from other hosts. Although new antigenic variants of the influenza A virus replace formerly circulating seasonal and pandemic viruses, replacement mechanisms remain poorly understood.
  
  
  METHODS
  A stochastic individual-based SEIR (susceptible-exposed-infectious-recovered) model with two viral strains (formerly circulating old strain and newly emerged strain) was developed for simulations to elucidate the replacement mechanisms.
  
  
  RESULTS
  Factors and conditions of virus and host populations affecting the replacement were identified. Replacement is more likely to occur in tropical regions than temperate regions. The magnitude of the ongoing epidemic by the old strain, herd immunity against the old strain, and timing of appearance of the new strain are not that important for replacement. It is probable that the frequency of replacement by a pandemic virus is higher than a seasonal virus because of the high initial susceptibility and high basic reproductive number of the pandemic virus.
  
  
  CONCLUSIONS
  The findings of this study on replacement mechanisms could lead to a better understanding of virus transmission dynamics and may possibly be helpful in establishing an effective strategy to mitigate the impact of seasonal and pandemic influenza.
authors: Department of Virology, Tohoku University Graduate School of Medicine, 2-1 Seiryo-machi, Aoba-ku, Sendai, Japan. Electronic address: furusey.tohoku@gmail.com.
authors: Yuki
authors: Y
authors: Furuse
authors: Department of Virology, Tohoku University Graduate School of Medicine, 2-1 Seiryo-machi, Aoba-ku, Sendai, Japan.
authors: Hitoshi
authors: H
authors: Oshitani
mesh_terms: D000940:Antigenic Variation; D003198:Computer Simulation; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology; D009980:Influenza A virus / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology / Q000821:virology*; D058873:Pandemics; D012621:Seasons
keywords: Antigenic drift; Ecology; Influenza; Pandemic; Population; Transmission dynamics