Influenza vaccine effectiveness among 50–64-year-old persons during a season of poor antigenic match between vaccine and circulating influenza virus strains: Colorado, United States, 2003–2004

During the 2003–2004 influenza season, we conducted a case–control study of influenza vaccine effectiveness (VE) among Colorado residents aged 50–64 years. Cases (n = 330) were identified from laboratory-confirmed influenza reports to the Colorado Department of Public Health and Environment (CDPHE). Controls (n = 1055) were recruited by random-digit dial telephone survey. VE was 60% (43–72%) and 48% (21–66%) among those without and with high-risk medical conditions, respectively. VE was 90% (68–97%) and 36% (0–63%) against influenza-related hospitalization for persons without and with high-risk conditions, respectively.