title: Usefulness of health registries when estimating vaccine effectiveness during the influenza A(H1N1)pdm09 pandemic in Norway.
abstract: BACKGROUND
  During the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm09, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). 2.2 million persons (45% of the population) were vaccinated with an AS03-adjuvanted monovalent vaccine during the pandemic. Most of them were registered in the Norwegian Immunisation Registry (SYSVAK). Based on these registries, we aimed at estimating the vaccine effectiveness (VE) and describing vaccine failures during the pandemic in Norway, in order to evaluate the role of the vaccine as a preventive measure during the pandemic.
  
  
  METHODS
  We conducted a population-based retrospective cohort study, linking MSIS and SYSVAK with pandemic influenza vaccination as exposure and laboratory-confirmed pandemic influenza as outcome. We measured VE by week and defined two thresholds for immunity; eight and 15 days after vaccination.
  
  
  RESULTS
  The weekly VE ranged from 77% to 96% when considering 15 days or more after vaccination as the threshold of immunity and from 73% to 94% when considering eight days or more. Overall, 157 individuals contracted pandemic influenza eight or more days after vaccination (8.4/100,000 vaccinated), of these 58 had onset 15 days or more after vaccination (3.0/100,000 vaccinated). Most of the vaccine failures occurred during the first weeks of the vaccination campaign. More than 30% of the vaccine failures were found in people below 10 years of age.
  
  
  CONCLUSIONS
  Having available health registries with data regarding cases of specific disease and vaccination makes it feasible to estimate VE in a simple and rapid way. VE was high regardless the immunity threshold chosen. We encourage public health authorities in other countries to set up such registries. It is also important to consider including information on underlying diseases in registries already existing, in order to make it feasible to conduct more complete VE estimations.
authors.affiliation: Division of Infectious Disease Control, Norwegian Institute of Public Health, Oslo, Norway. BernardoRafael.Guzman.Herrador@fhi.no
authors.forename: Bernardo Rafael
authors.initials: BR
authors.lastname: Guzmán Herrador
authors.forename: Preben
authors.initials: P
authors.lastname: Aavitsland
authors.forename: Berit
authors.initials: B
authors.lastname: Feiring
authors.forename: Marianne A
authors.initials: MA
authors.lastname: Riise Bergsaker
authors.forename: Katrine
authors.initials: K
authors.lastname: Borgen
mesh_terms: D000276:Adjuvants, Immunologic / Q000008:administration & dosage; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004338:Drug Combinations; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control* / Q000821:virology; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D011136:Polysorbates / Q000008:administration & dosage; D012042:Registries / Q000706:statistics & numerical data*; D012189:Retrospective Studies; D013185:Squalene / Q000008:administration & dosage; D016896:Treatment Outcome; D055815:Young Adult; D024502:alpha-Tocopherol / Q000008:administration & dosage
chemical_list: D000276:Adjuvants, Immunologic; D004338:Drug Combinations; D007252:Influenza Vaccines; D011136:Polysorbates; D013185:Squalene; C550253:AS03 adjuvant; D024502:alpha-Tocopherol