Relative vaccine efficacy of high-dose versus standard-dose influenza vaccines in preventing probable influenza in a Medicare Fee-for-Service population.

BACKGROUND

High-dose (HD) influenza vaccine, currently the most commonly used vaccine among US seniors (aged ≥ 65 years), has been shown to be more efficacious than standard-dose (SD) vaccine in multiple randomized trials. This study evaluated the real-world relative vaccine effectiveness (rVE) of HD vs SD over four influenza seasons.

METHODS

This study included Medicare Fee-for-Service enrollees who received HD or SD at an outpatient clinic or pharmacy during influenza seasons 2011-2012 through 2014-2015. Probable influenza (an inpatient stay with an influenza diagnosis on the claim, or an outpatient visit with a rapid influenza test/culture followed by an antiviral prescription) was assessed among HD recipients matched 1:1 with SD recipients by location, vaccination date, age, and sex. Fine-Gray subdistribution hazard models with competing risk of death were used to adjust for residual confounding. Analyses were stratified by outpatient vs pharmacy vaccination.

RESULTS

Across the four influenza seasons, there were 535,598, 1,017,552, 1,548,164, and 2,420,450 in the pharmacy cohort; and 821,662, 1,151,080, 1,559,488, and 2,421,758 in the outpatient cohort. During peak influenza season, rVEs for 2011-12 through 2014-15 were 21.8% (95% CI: -5.9%, 42.3%), 14.8% (9.3%, 19.9%), 16.9% (9.2%, 23.9%), and 17.2% (14.5%, 19.9%), respectively, in the pharmacy cohort; and 16.5% (-5.9%, 34.2%), 15.1% (10.9%, 19.1%), 10.0% (2.9%, 16.6%), and -0.2% (-3.0%, 2.5%), respectively, in the outpatient cohort.

CONCLUSION

HD was consistently associated with better protection against probable influenza. The lower treatment effect observed in the outpatient cohort could reflect provider bias due to physicians triaging HD to frailer patients.