Data on influenza vaccine effectiveness (IVE) against mortality is 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.
Data were collected between 2010–2017 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 were included, and a subset of test-negative controls. Propensity score analysis and multivariable logistic regression were used to determine the adjusted Odds Ratio (aOR) of vaccination, with IVE= 1–aOR x 100%. Residual confounding was assessed by examining mortality in controls.
Over eight 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%CI: 3–51%, p=0.033) 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=0.031). There was no evidence that vaccination reduced the severity of influenza illness (aOR: 1.07 (95%CI: 0.76–1.50); p=0.713).
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.