Influenza vaccine effectiveness in general practice and in hospital patients in Victoria, 2011-2013

Objective: To compare influenza vaccine effectiveness in the general practice and hospital settings. Design: Analysis of annual case test-negative studies. Setting: Victorian sentinel hospitals and general practices, 2011-2013. Participants: Patients presenting to general practitioners, or those admitted to hospital with an influenza-like illness who were tested for influenza using a polymerase chain reaction assay. Cases were patients with a positive test result for influenza; non-cases (controls) had a negative test result. Main outcome measures: Vaccine effectiveness against laboratory-confirmed influenza. Results: Hospitalised patients were on average older and reported a higher proportion of comorbidities than general practice patients. The pooled estimate of influenza vaccine effectiveness against laboratory-confirmed infection for the 3 years was 50% (95% CI, 26%-66%) for general practice patients and 39% (95% CI, 28%-47%) for patients admitted to hospital. Conclusions: Influenza vaccines appeared to be similarly modestly effective in the general practice and hospital settings. Influenza vaccination appears to prevent hospital admission by preventing symptomatic infection rather than by attenuating the severity of illness.