title: Vaccine effectiveness against laboratory-confirmed influenza hospitalizations among elderly adults during the 2010-2011 season.
abstract: BACKGROUND
  Although annual influenza immunization is recommended for adults aged ≥65 years due to the substantial burden of illness, the evidence base for this recommendation is weak. Prior observational studies that examined influenza vaccine effectiveness against nonspecific serious outcomes suffered from selection bias and the lack of laboratory confirmation for influenza infection. The objective of this study was to determine the effectiveness of the 2010-2011 seasonal influenza vaccine against laboratory-confirmed influenza hospitalizations among community-dwelling elderly adults, a serious and highly specific outcome.
  
  
  METHODS
  We conducted a test-negative study of community-dwelling adults aged >65 years in Ontario, Canada. Respiratory specimens collected between 1 December 2010 and 30 April 2011 from patients admitted to acute care hospitals were tested for influenza using nucleic acid amplification techniques. Influenza vaccination was ascertained from physician billing claims through linkage to health administrative datasets.
  
  
  RESULTS
  Receipt of the 2010-2011 seasonal influenza vaccine was associated with a 42% (95% confidence interval, 29%-53%) reduction in laboratory-confirmed influenza hospitalizations. Vaccine effectiveness estimates were consistent across age groups, by sex, and regardless of outcome severity, timing of testing, and when considering individuals vaccinated <7 or <14 days prior to admission as unvaccinated.
  
  
  CONCLUSIONS
  Results of this study will better inform decision making regarding influenza vaccination of elderly adults. Similar analyses are needed annually due to antigenic drift and frequent changes in influenza vaccine composition. The linkage of routinely collected laboratory testing and health administrative data represents an efficient method for estimating influenza vaccine effectiveness that complements prospective studies.
authors.affiliation: Institute for Clinical Evaluative Sciences, 2075 Bayview Ave, Toronto, Ontario, Canada. jeff.kwong@utoronto.ca
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Kwong
authors.forename: Michael A
authors.initials: MA
authors.lastname: Campitelli
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Gubbay
authors.forename: Adriana
authors.initials: A
authors.lastname: Peci
authors.forename: Anne-Luise
authors.initials: AL
authors.lastname: Winter
authors.forename: Romy
authors.initials: R
authors.lastname: Olsha
authors.forename: Robert
authors.initials: R
authors.lastname: Turner
authors.forename: Laura C
authors.initials: LC
authors.lastname: Rosella
authors.forename: Natasha S
authors.initials: NS
authors.lastname: Crowcroft
mesh_terms: D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control*; D016015:Logistic Models; D008297:Male; D009864:Ontario / Q000453:epidemiology
chemical_list: D007252:Influenza Vaccines
keywords: elderly adults; hospitalization; influenza vaccine; vaccine effectiveness