title: Multiple infections with seasonal influenza A virus induce cross-protective immunity against A(H1N1) pandemic influenza virus in a ferret model.
abstract: BACKGROUND
  An age bias toward children and young adults has been reported for infection and hospitalizations with pandemic H1N1 influenza (A[H1N1]pdm) in the 2009 and 2010 influenza seasons in the Southern and Northern Hemispheres. Serological analysis of prepandemic samples has shown a higher incidence of cross-reactive antibodies to A(H1N1)pdm virus in older populations; conserved T cell epitopes between viruses have been identified. The contribution of preexisting immunity to seasonal influenza to protection against A(H1N1)pdm infection was analyzed in a ferret model.
  
  
  METHODS
  Ferrets were pre-infected with influenza A viruses and/or vaccinated with inactivated influenza viruses with adjuvant. Infection after challenge was assessed by measuring shedding virus, transmission to naive animals, and seroconversion.
  
  
  RESULTS
  Homologous vaccination reduced the incidence of infection and delayed transmission. Pre-infection with virus induced sterilizing immunity to homologous challenge. One prior infection with seasonal influenza A virus improved clearance of A(H1N1)pdm virus. Prior infection with A(H1N1)pdm virus reduced shedding after seasonal influenza A challenge. Two infections with seasonal influenza A viruses reduced the incidence of infection, the amount and duration of virus shedding, and the frequency of transmission following A(H1N1)pdm challenge.
  
  
  CONCLUSIONS
  These data suggest the reduced incidence and severity of infection with A(H1N1)pdm virus in the adult population during the 2009-2010 influenza season may be a result of previous exposure to seasonal influenza A viruses.
authors: World Health Organization Collaborating Centre for Reference and Research on Influenza, Victorian Infectious Diseases Reference Laboratory, North Melbourne, Australia. karen.laurie@influenzacentre.org
authors: Karen L
authors: KL
authors: Laurie
authors: Louise A
authors: LA
authors: Carolan
authors: Deborah
authors: D
authors: Middleton
authors: Sue
authors: S
authors: Lowther
authors: Anne
authors: A
authors: Kelso
authors: Ian G
authors: IG
authors: Barr
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000097:blood* / Q000276:immunology*; D056738:Cross Protection; D005289:Ferrets; D007117:Immunization, Secondary / Q000379:methods; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007252:Influenza Vaccines / Q000276:immunology*; D009976:Orthomyxoviridae Infections / Q000276:immunology / Q000821:virology; D014611:Vaccination / Q000379:methods
chemical_list: D000914:Antibodies, Viral; D007252:Influenza Vaccines