Influenza Vaccine Effectiveness Against Influenza-Related Mortality in Australian Hospitalized Patients: A Propensity Score Analysis

BACKGROUND: Data on influenza vaccine effectiveness (IVE) against mortality are limited, with no Australian data to guide vaccine uptake. We aimed to assess IVE against influenza-related mortality in Australian hospitalized patients, assess residual confounding in the association between influenza vaccination and mortality, and assess whether influenza vaccination reduces the severity of influenza illness. METHODS: Data were collected between 2010 and 2017 from a national Australian hospital-based sentinel surveillance system using a case-control design. Adults and children admitted to the 17 study hospitals with acute respiratory symptoms were tested for influenza using nucleic acid testing; all eligible test-positive cases, and a subset of test-negative controls, were included. Propensity score analysis and multivariable logistic regression were used to determine the adjusted odds ratio (aOR) of vaccination, with IVE = 1 - aOR × 100%. Residual confounding was assessed by examining mortality in controls. RESULTS: Over 8 seasons, 14038 patients were admitted with laboratory-confirmed influenza. The primary analysis included 9298 cases and 6451 controls, with 194 cases and 136 controls dying during hospitalization. Vaccination was associated with a 31% (95% confidence interval [CI], 3%-51%; P = .033) reduction in influenza-related mortality, with similar estimates in the National Immunisation Program target group. Residual confounding was identified in patients ≥65 years old (aOR, 1.92 [95% CI, 1.06-3.46]; P = .031). There was no evidence that vaccination reduced the severity of influenza illness (aOR, 1.07 [95% CI, .76-1.50]; P = .713). CONCLUSIONS: Influenza vaccination is associated with a moderate reduction in influenza-related mortality. This finding reinforces the utility of the Australian vaccination program in protecting those most at risk of influenza-related deaths.