So how good is the flu shot really?/ Clinical Infectious Disease

CDC says flu vaccines prevent flu, on average 40% of the time, but even if you do get flu, the vaccine makes it less severe.  But when these two claims were tested in Australia, neither was found to be true.  The Australian study found  that (in 17 hospitals over 7 recent flu seasons) there was no reduction in flu symptom severity in those who had been vaccinated versus those who had not.

The study also found, using a case-control design, that the vaccine averaged 31% effectiveness at preventing the flu.  Earlier flu studies have been rightly criticized for ignoring the "healthy vaccinee"  effect.  The healthy vaccinee effect refers to the fact that, overall, elders who receive flu vaccine in the US are a healthier group than elders who do not receive flu vaccine... possibly because their doctors felt they were too frail to risk a side effect.  If the authors did not take this factor into account (and the whole article is behind a paywall so I could not read the details) then the actual efficacy might be considerably lower.

Influenza Vaccine Effectiveness against influenza-related mortality in Australian hospitalized patients:  a propensity score analysis.

Monica L Nation, Robert Moss, Matthew J Spittal, Tom Kotsimbos, Paul M Kelly, Allen C Cheng on behalf of the FluCAN Investigators

Clinical Infectious Diseases,  https://doi.org/10.1093/cid/ciz1238

Published:  06 January 2020

Background

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.

Methods

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.

Results

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).

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.