title: Further evidence for bias in observational studies of influenza vaccine effectiveness: the 2009 influenza A(H1N1) pandemic.
abstract: Preinfluenza periods have been used to test for uncontrolled confounding in studies of influenza vaccine effectiveness, but some authors have claimed that confounding differs in preinfluenza and influenza periods. We tested this claim by comparing estimates of the vaccine-mortality association during the 2009/2010 influenza year, when there was essentially no circulation of seasonal influenza in the United States, and 2007/2008, a typical influenza year. We pooled data on seniors (adults aged ≥65 years) from 7 US managed care organizations that participated in the Vaccine Safety Datalink Project. We defined influenza vaccination, all-cause mortality, and potential confounders from administrative databases. We quantified the vaccine-mortality association using Cox regression. During 2007/2008, the adjusted hazard ratio was 0.44 prior to influenza season, 0.62 during influenza season, and 0.71 after influenza season. A similar pattern was observed during 2009/2010, when any effect of seasonal influenza vaccine observed during all time periods must have resulted from confounding: 0.65 during the autumn, 0.80 during the winter, and 0.84 during the summer. In a year with minimal seasonal influenza, we found no evidence that confounding in autumn preinfluenza periods is qualitatively different from confounding in winter. This supports the use of preinfluenza periods as control time periods in studies of influenza vaccine effectiveness.
authors: Michael L
authors: ML
authors: Jackson
authors: Onchee
authors: O
authors: Yu
authors: Jennifer C
authors: JC
authors: Nelson
authors: Allison
authors: A
authors: Naleway
authors: Edward A
authors: EA
authors: Belongia
authors: Roger
authors: R
authors: Baxter
authors: Komal
authors: K
authors: Narwaney
authors: Steven J
authors: SJ
authors: Jacobsen
authors: David K
authors: DK
authors: Shay
authors: Lisa A
authors: LA
authors: Jackson
mesh_terms: D000368:Aged; D015982:Bias; D015986:Confounding Factors, Epidemiologic; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007252:Influenza Vaccines / Q000276:immunology; D007251:Influenza, Human / Q000453:epidemiology / Q000517:prevention & control*; D008297:Male; D009026:Mortality; D019370:Observation; D058873:Pandemics; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D012621:Seasons; D014481:United States / Q000453:epidemiology
chemical_list: D007252:Influenza Vaccines
keywords: confounding factors (epidemiology); epidemiologic methods; influenza; influenza, pandemic; vaccine effectiveness; vaccines
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