title: New strategies are needed to improve the accuracy of influenza vaccine effectiveness estimates among seniors.
abstract: OBJECTIVE
  The magnitude of the benefit of influenza vaccine among elderly individuals has been recently debated. Existing vaccine effectiveness estimates derive primarily from observational studies, which may be biased. In this paper, we provide a methodological examination of the potential sources of bias in observational studies of influenza vaccine effectiveness in seniors and propose design and analysis strategies to reduce bias in future studies.
  
  
  METHODS
  We draw parallels to bias documented in observational studies of therapies in other areas of medical research including pharmacoepidemiology, discuss reasons why existing adjustment methods in influenza studies may not adequately control for the bias, and evaluate statistical approaches that may yield more accurate estimation of influenza vaccine effectiveness.
  
  
  RESULTS
  There is strong evidence for the presence of bias in existing observational estimates of influenza vaccine effectiveness in the elderly and the failure of current adjustment methods to reduce bias.
  
  
  CONCLUSIONS
  Promising approaches for reducing bias include obtaining more accurate information on confounders, such as functional status, avoiding all-cause death in favor of outcomes, such as pneumonia or influenza-related pneumonia, and evaluating the extent to which bias is reduced by these and other methods using the 'control' period before influenza season.
authors.affiliation: Group Health Center for Health Studies, Seattle, WA 98101, USA. nelson.jl@ghc.org
authors.forename: Jennifer Clark
authors.initials: JC
authors.lastname: Nelson
authors.forename: Michael L
authors.initials: ML
authors.lastname: Jackson
authors.forename: Noel S
authors.initials: NS
authors.lastname: Weiss
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Jackson
mesh_terms: D000368:Aged; D015982:Bias; D015986:Confounding Factors, Epidemiologic; D003627:Data Interpretation, Statistical; D019317:Evidence-Based Medicine / Q000592:standards; D006801:Humans; D017589:Immunization Programs; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000517:prevention & control*; D011634:Public Health; D012107:Research Design; D016896:Treatment Outcome; D014611:Vaccination
chemical_list: D007252:Influenza Vaccines