title: Seroprevalence of antibody to influenza A(H1N1)pdm09 attributed to vaccination or infection, before and after the second (2010) pandemic wave in Australia.
abstract: OBJECTIVE
  Historical records of influenza pandemics demonstrate variability in incidence and severity between waves. The influenza A(H1N1)pdm09 pandemic was the first in which many countries implemented strain-specific vaccination to mitigate subsequent seasons. Serosurveys provide opportunity to examine the constraining influence of antibody on population disease experience.
  
  
  METHODS
  Changes in the proportion of adults seropositive to influenza A(H1N1)pdm09over the 2009/10 (summer) interepidemic period and 2010 (winter) influenza season were measured to determine whether there was a temporal relationship with vaccine distribution and influenza activity, respectively.
  
  
  METHODS
  Australia.
  
  
  METHODS
  Plasma samples were collected from healthy blood donors from seven cities at the end of the first wave (November 2009), and before (March/April 2010) and after (November 2010) the subsequent influenza season.
  
  
  METHODS
  Haemagglutination inhibition (HI) assays were performed to assess reactivity of plasma against A(H1N1)pdm09, and the proportion seropositive (HI titre ≥ 40) compared over time, by age group and location.
  
  
  RESULTS
  Between the 2009 and 2010 influenza seasons, the seropositive proportion rose from 22% to 43%, an increase observed across all ages and sites. Brisbane alone recorded a significant rise in seropositivity over the 2010 influenza season - from a baseline of 35% to 53%. The seropositive proportion elsewhere was ≥40% pre-season, and did not rise over winter.
  
  
  CONCLUSIONS
  A vaccine-associated increase in seropositive proportion preceding the influenza season correlated with low levels of disease activity in winter 2010. These observations support the role of immunisation in mitigating the 'second wave' of A(H1N1)pdm09, with timing critical to ensure sustained herd protection.
authors.affiliation: Vaccine & Immunisation Research Group, Murdoch Children's Research Institute, Melbourne School of Population and Global Health, the University of Melbourne, Parkville, Vic., Australia; Victorian Infectious Diseases Reference Laboratory, North Melbourne, Vic., Australia.
authors.forename: Jodie
authors.initials: J
authors.lastname: McVernon
authors.forename: Karen
authors.initials: K
authors.lastname: Laurie
authors.forename: Helen
authors.initials: H
authors.lastname: Faddy
authors.forename: David
authors.initials: D
authors.lastname: Irving
authors.forename: Terry
authors.initials: T
authors.lastname: Nolan
authors.forename: Ian
authors.initials: I
authors.lastname: Barr
authors.forename: Anne
authors.initials: A
authors.lastname: Kelso
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral / Q000097:blood*; D001315:Australia / Q000453:epidemiology; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D016036:Seroepidemiologic Studies; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral
keywords: Blood donors; immunity, herd; influenza, human; pandemics; serology; vaccination