Estimating influenza vaccine effectiveness in an outbreak when anti-viral medications were used as a control measure

Objectives: To estimate influenza vaccine effectiveness (VE) from an outbreak of influenza A in an aged care facility in which neuraminidase inhibitors were used as part of outbreak control measures. Methods: The outbreak occurred between 25 December 2001 and 21 January 2002. Neuraminidase inhibitors were used to control the outbreak. Residents and staff with respiratory symptoms were tested for influenza using RT-PCR and/or serology. Vaccine effectiveness (VE) was estimated for the prevention of laboratory-confirmed influenza. Results: Nineteen of 42 (45%) residents and four of 29 (14%) staff were laboratory-confirmed as cases. The outbreak was caused by an influenza A (H3N2) strain, antigenically matched to that season's vaccine. The VE for preventing laboratory-confirmed influenza infection was 61% (95% CI 41-73) among residents and 100% (95% CI 63-100) among staff. Conclusions: The VE estimates calculated from this outbreak were consistent with other published results. Outbreaks of influenza in institutions provide a good opportunity to review influenza VE, but use of antiviral medications as control measures may affect interpretation of findings.