• Michael L Anderson, Carlos Dobkin, and Devon Gorry.
• University of California, Berkeley, Berkeley, California, and National Bureau of Economic Research, Cambridge, Massachusetts (M.L.A.).
• Ann. Intern. Med. 2020 Apr 7; 172 (7): 445-452.

BackgroundObservational studies using traditional research designs suggest that influenza vaccination reduces hospitalizations and mortality among elderly persons. Accordingly, health authorities in some countries prioritize vaccination of this population. Nevertheless, questions remain about this policy's effectiveness given the potential for bias and confounding in observational data.ObjectiveTo determine the effectiveness of the influenza vaccine in reducing hospitalizations and mortality among elderly persons by using an observational research design that reduces the possibility of bias and confounding.DesignA regression discontinuity design was applied to the sharp change in vaccination rate at age 65 years that resulted from an age-based vaccination policy in the United Kingdom. In this design, comparisons were limited to individuals who were near the age-65 threshold and were thus plausibly similar along most dimensions except vaccination rate.SettingEngland and Wales.ParticipantsAdults aged 55 to 75 years residing in the study area during 2000 to 2014.InterventionSeasonal influenza vaccine.MeasurementsHospitalization and mortality rates by month of age.ResultsThe data included 170 million episodes of care and 7.6 million deaths. Turning 65 was associated with a statistically and clinically significant increase in rate of seasonal influenza vaccination. However, no evidence indicated that vaccination reduced hospitalizations or mortality among elderly persons. The estimates were precise enough to rule out results from many previous studies.LimitationThe study relied on observational data, and its focus was limited to individuals near age 65 years.ConclusionCurrent vaccination strategies prioritizing elderly persons may be less effective than believed at reducing serious morbidity and mortality in this population, which suggests that supplementary strategies may be necessary.Primary Funding SourceNational Institute on Aging.

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