title: Effectiveness of pandemic and seasonal influenza vaccines in preventing laboratory-confirmed influenza in adults: a clinical cohort study during epidemic seasons 2009-2010 and 2010-2011 in Finland.
abstract: BACKGROUND
  One dose of pandemic influenza vaccine Pandemrix (GlaxoSmithKline) was offered to the entire population of Finland in 2009-10. We conducted a prospective clinical cohort study to determine the vaccine effectiveness in preventing febrile laboratory-confirmed influenza infection during the influenza season 2009-10 and continued the study in 2010-11.
  
  
  METHODS
  In total, 3,518 community dwelling adults aged 18-75 years living in Tampere city were enrolled. The participants were not assigned to any vaccination regimen, but they could participate in the study regardless of their vaccination status or intention to be vaccinated with the pandemic or seasonal influenza vaccine. They were asked to report if they received Pandemrix in 2009-10 and/or trivalent influenza vaccine in 2010-11. Vaccinations were verified from medical records. The participants were instructed to report all acute symptoms of respiratory tract infection with fever (at least 38°C) and pneumonias to the study staff. Nasal and oral swabs were obtained within 5-7 days after symptom onset and influenza-specific RNA was identified by reverse transcription polymerase chain reaction.
  
  
  RESULTS
  In 2009-10, the estimated vaccine effectiveness was 81% (95%CI 30-97). However, the vaccine effectiveness could not be estimated reliably, because only persons in prioritized groups were vaccinated before/during the first pandemic wave and many participants were enrolled when they already had the symptoms of A(H1N1)pdm09 influenza infection. In 2010-11, 2,276 participants continued the follow-up. The vaccine effectiveness, adjusted for potential confounding factors was 81% (95%CI 41-96) for Pandemrix only and 88% (95%CI 63-97) for either Pandemrix or trivalent influenza vaccine 2010-11 or both, respectively.
  
  
  CONCLUSIONS
  Vaccination with an AS03-adjuvanted pandemic vaccine in 2009-10 was still effective in preventing A(H1N1)pdm09 influenza during the following epidemic season in 2010-11.
  
  
  BACKGROUND
  ClinicalTrials.gov NCT01024725. NCT01206114.
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Tampere, Finland.
authors.forename: Ritva K
authors.initials: RK
authors.lastname: Syrjänen
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Jokinen
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Thedi
authors.initials: T
authors.lastname: Ziegler
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Jonas
authors.initials: J
authors.lastname: Sundman
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Mika
authors.initials: M
authors.lastname: Lahdenkari
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Ilkka
authors.initials: I
authors.lastname: Julkunen
authors.affiliation: Department of Vaccination and Immune Protection, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Terhi M
authors.initials: TM
authors.lastname: Kilpi
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology; D007252:Influenza Vaccines / Q000627:therapeutic use*; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D014611:Vaccination; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines; C556153:pandemrix