title: Pandemic influenza due to pH1N1/2009 virus: estimation of infection burden in Reunion Island through a prospective serosurvey, austral winter 2009.
abstract: BACKGROUND
  To date, there is little information that reflects the true extent of spread of the pH1N1/2009v influenza pandemic at the community level as infection often results in mild or no clinical symptoms. This study aimed at assessing through a prospective study, the attack rate of pH1N1/2009 virus in Reunion Island and risk factors of infection, during the 2009 season.
  
  
  RESULTS
  A serosurvey was conducted during the 2009 austral winter, in the frame of a prospective population study. Pairs of sera were collected from 1687 individuals belonging to 772 households, during and after passage of the pandemic wave. Antibodies to pH1N1/2009v were titered using the hemagglutination inhibition assay (HIA) with titers ≥ 1/40 being considered positive. Seroprevalence during the first two weeks of detection of pH1N1/2009v in Reunion Island was 29.8% in people under 20 years of age, 35.6% in adults (20-59 years) and 73.3% in the elderly (≥ 60 years) (P<0.0001). Baseline corrected cumulative incidence rates, were 42.9%, 13.9% and 0% in these age groups respectively (P<0.0001). A significant decline in antibody titers occurred soon after the passage of the epidemic wave. Seroconversion rates to pH1N1/2009 correlated negatively with age: 63.2%, 39.4% and 16.7%, in each age group respectively (P<0.0001). Seroconversion occurred in 65.2% of individuals who were seronegative at inclusion compared to 6.8% in those who were initially seropositive.
  
  
  CONCLUSIONS
  Seroincidence of pH1N1/2009v infection was three times that estimated from clinical surveillance, indicating that almost two thirds of infections occurring at the community level have escaped medical detection. People under 20 years of age were the most affected group. Pre-epidemic titers ≥ 1/40 prevented seroconversion and are likely protective against infection. A concern was raised about the long term stability of the antibody responses.
authors: GIS CRVOI, Centre de Recherche et de Veille sur les Maladies Emergentes dans l'Océan Indien, Saint-Denis, La Réunion. koussay.dellagi@ird.fr
authors: Koussay
authors: K
authors: Dellagi
authors: Olivier
authors: O
authors: Rollot
authors: Sarah
authors: S
authors: Temmam
authors: Nicolas
authors: N
authors: Salez
authors: Vanina
authors: V
authors: Guernier
authors: Hervé
authors: H
authors: Pascalis
authors: Patrick
authors: P
authors: Gérardin
authors: Adrian
authors: A
authors: Fianu
authors: Nathanael
authors: N
authors: Lapidus
authors: Nadège
authors: N
authors: Naty
authors: Pablo
authors: P
authors: Tortosa
authors: Karim
authors: K
authors: Boussaïd
authors: Marie-Christine
authors: MC
authors: Jaffar-Banjee
authors: Laurent
authors: L
authors: Filleul
authors: Antoine
authors: A
authors: Flahault
authors: Fabrice
authors: F
authors: Carrat
authors: Francois
authors: F
authors: Favier
authors: Xavier
authors: X
authors: de Lamballerie
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D005602:France / Q000453:epidemiology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000276:immunology / Q000472:pathogenicity*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology* / Q000276:immunology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D016133:Polymerase Chain Reaction; D011446:Prospective Studies; D012621:Seasons; D012698:Serologic Tests; D055815:Young Adult