title: Incidence of H1N1 2009 virus infection through the analysis of paired plasma specimens among blood donors, France.
abstract: BACKGROUND
  Knowledge of the age-specific prevalence of seroprotection and incidence of seroconversion infection is necessary to complement clinical surveillance data and statistical models. It provides the basis for estimating the future impact of influenza A (H1N1pdm09) and implementing appropriate prevention and response strategies.
  
  
  METHODS
  Using a cross-sectional design, two-stage stratified sampling and paired plasma samples, we estimated the age-specific prevalence of a protective level of H1N1pdm09 antibodies in the French adult population before and after the 2009/10 pandemic, and the proportion of those susceptible that seroconverted due to infection, from a single sample of 1,936 blood donors aged 20-70 years in mainland France in June 2010. Samples with a haemagglutination inhibition (HI) titre ≥1∶40 were considered seropositive, and seroconversion due to infection was defined as a 4-fold increase in titre in the absence of H1N1pdm09 vaccination or pre-pandemic seropositivity.
  
  
  RESULTS
  Out of the 1,936 donors, 1,708 were included in the analysis. Seroprevalence before the pandemic was 6.7% (95% CI 5.0, 8.9) with no significant differences by age-group (p = 0.3). Seroprevalence afterwards was 23.0% (95% CI 17.7, 29.3) with 20-29 year olds having a higher level than older groups (p<0.001). Seroconversion due to infection was 12.2% (95% CI 6.9, 20.5). Younger age-group, vaccination against H1N1 and being seropositive before the pandemic were strongly associated with post-pandemic seropositivity.
  
  
  CONCLUSIONS
  Before the 2009/2010 winter influenza season, only 6.7% of the French mainland population aged 20-70 had a level of antibodies usually considered protective. During the first pandemic wave, 12.2% of the population seroconverted due to infection and the seroprevalence after the wave rose to 23%, either due to prepandemic seropositivity, infection or vaccination. This relatively low latter figure contributed to an extension of target groups for influenza vaccination for the 2010/2011 season.
authors.affiliation: Institut de Veille Sanitaire, St Maurice, France. angie.bone@hpa.org.uk
authors.forename: Angie
authors.initials: A
authors.lastname: Bone
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Guthmann
authors.forename: Azzedine
authors.initials: A
authors.lastname: Assal
authors.forename: Dominique
authors.initials: D
authors.lastname: Rousset
authors.forename: Armelle
authors.initials: A
authors.lastname: Degeorges
authors.forename: Pascal
authors.initials: P
authors.lastname: Morel
authors.forename: Martine
authors.initials: M
authors.lastname: Valette
authors.forename: Vincent
authors.initials: V
authors.lastname: Enouf
authors.forename: Eric
authors.initials: E
authors.lastname: Jacquot
authors.forename: Bertrand
authors.initials: B
authors.lastname: Pelletier
authors.forename: Yann
authors.initials: Y
authors.lastname: Le Strat
authors.forename: Josiane
authors.initials: J
authors.lastname: Pillonel
authors.forename: Laure
authors.initials: L
authors.lastname: Fonteneau
authors.forename: Sylvie
authors.initials: S
authors.lastname: van der Werf
authors.forename: Bruno
authors.initials: B
authors.lastname: Lina
authors.forename: Pierre
authors.initials: P
authors.lastname: Tiberghien
authors.forename: Daniel
authors.initials: D
authors.lastname: Lévy-Bruhl
mesh_terms: D000328:Adult; D000368:Aged; D001782:Blood Donors; D003430:Cross-Sectional Studies; D005260:Female; D005602:France / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D016036:Seroepidemiologic Studies