title: Field effectiveness of pandemic and 2009-2010 seasonal vaccines against 2009-2010 A(H1N1) influenza: estimations from surveillance data in France.
abstract: BACKGROUND
  In this study, we assess how effective pandemic and trivalent 2009-2010 seasonal vaccines were in preventing influenza-like illness (ILI) during the 2009 A(H1N1) pandemic in France. We also compare vaccine effectiveness against ILI versus laboratory-confirmed pandemic A(H1N1) influenza, and assess the possible bias caused by using non-specific endpoints and observational data.
  
  
  RESULTS
  We estimated vaccine effectiveness by using the following formula: VE  =  (PPV-PCV)/(PPV(1-PCV)) × 100%, where PPV is the proportion vaccinated in the population and PCV the proportion of vaccinated influenza cases. People were considered vaccinated three weeks after receiving a dose of vaccine. ILI and pandemic A(H1N1) laboratory-confirmed cases were obtained from two surveillance networks of general practitioners. During the epidemic, 99.7% of influenza isolates were pandemic A(H1N1). Pandemic and seasonal vaccine uptakes in the population were obtained from the National Health Insurance database and by telephonic surveys, respectively. Effectiveness estimates were adjusted by age and week. The presence of residual biases was explored by calculating vaccine effectiveness after the influenza period. The effectiveness of pandemic vaccines in preventing ILI was 52% (95% confidence interval: 30-69) during the pandemic and 33% (4-55) after. It was 86% (56-98) against confirmed influenza. The effectiveness of seasonal vaccines against ILI was 61% (56-66) during the pandemic and 19% (-10-41) after. It was 60% (41-74) against confirmed influenza.
  
  
  CONCLUSIONS
  The effectiveness of pandemic vaccines in preventing confirmed pandemic A(H1N1) influenza on the field was high, consistently with published findings. It was significantly lower against ILI. This is unsurprising since not all ILI cases are caused by influenza. Trivalent 2009-2010 seasonal vaccines had a statistically significant effectiveness in preventing ILI and confirmed pandemic influenza, but were not better in preventing confirmed pandemic influenza than in preventing ILI. This lack of difference might be indicative of selection bias.
authors.affiliation: UPMC Université Paris 06, UMRS 707, Paris, France. pelat@u707.jussieu.fr
authors.forename: Camille
authors.initials: C
authors.lastname: Pelat
authors.forename: Alessandra
authors.initials: A
authors.lastname: Falchi
authors.forename: Fabrice
authors.initials: F
authors.lastname: Carrat
authors.forename: Anne
authors.initials: A
authors.lastname: Mosnier
authors.forename: Isabelle
authors.initials: I
authors.lastname: Bonmarin
authors.forename: Clément
authors.initials: C
authors.lastname: Turbelin
authors.forename: Sophie
authors.initials: S
authors.lastname: Vaux
authors.forename: Sylvie
authors.initials: S
authors.lastname: van der Werf
authors.forename: Jean Marie
authors.initials: JM
authors.lastname: Cohen
authors.forename: Bruno
authors.initials: B
authors.lastname: Lina
authors.forename: Thierry
authors.initials: T
authors.lastname: Blanchon
authors.forename: Thomas
authors.initials: T
authors.lastname: Hanslik
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005602:France / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007252:Influenza Vaccines / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology* / Q000517:prevention & control / Q000821:virology; D008875:Middle Aged; D058873:Pandemics / Q000517:prevention & control*; D011159:Population Surveillance; D015203:Reproducibility of Results; D012621:Seasons; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines