The aim of this study was to estimate influenza vaccine effectiveness (IVE) against laboratory-confirmed influenza among hospitalized patients. A case-control investigation was based on the prospective surveillance of influenza-like illness (ILI) during five flu seasons. We compared influenza-positive cases and influenza-negative controls. Unadjusted overall IVE was 62% (95% confidence interval 24% to 81%). We found that IVE was lower during the 2004-05 flu season (11%; 95% CI -232% to 76%) when the vaccine and circulating viruses were mismatched. Expansion of the study to other hospitals could provide IVE estimates earlier in the season, for different age groups and emerging virus strains.
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