Influenza vaccine effectiveness among healthcare workers in comparison to hospitalized patients: A 2004-2009 case-test, negative-control, prospective study.

The objective of this study was to calculate Vaccine Effectiveness (VE) in healthcare workers (HCW) and to compare VE between patients and HCW. A case-control investigation based on the prospective study was conducted between 2004 and 2009 in a teaching hospital. All HCW with influenza-like illness (ILI) from participating units (n = 24) were included, and vaccination status was characterized by interview. A total of 150 HCW presented ILI; 130 (87%) were female, 27 (18%) were positive for influenza, and 42 (28%) were vaccinated. Adjusted VE was 89% (95% CI 39 to 98). Among patients, adjusted VE was 42% (95% CI -39 to 76). The difference of VE (VEhcw - VEpat) was 46.15% (95% CI 2.41 to 144). The VE ratio (VEhcw / VEpat) was 2.09 (95% CI -1.60 to 134.17). Influenza VE differed between HCW and patients when the flu season was taken into account. This finding confirms the major impact of host determinants on influenza VE.