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Background High dose (HD) influenza vaccine has been shown to be more efficacious than standard dose (SD) vaccine in multiple randomized trials. HD is currently the most commonly used vaccine in US seniors (≥65 years of age). In this study, we evaluated the real-world relative vaccine effectiveness (rVE) of HD vs SD over 3 influenza seasons. Methods This study includes a cohort of Medicare fee-for-service enrollees during influenza seasons 2011–2012 to 2013–2014 who received either HD or SD at a pharmacy or an outpatient clinic. HD recipients were matched 1:1 to SD recipients based on location, date of vaccination, age, and gender. Fine-Gray subdistribution hazard models with competing risk of death were used to adjust for residual confounding. The study outcome of probable influenza was defined as any inpatient stay with an influenza diagnosis on the claim, or an outpatient medical encounter with a rapid influenza test/culture followed by an antiviral prescription. Analyses were stratified based on vaccination location (clinic vs pharmacy) as it is expected that physicians carrying both vaccines may prioritize HD to frailer patients, while pharmacists may not exercise clinical judgment. Results Over the influenza seasons 2011–2012, 2012/–2013, and 2013–2014, 1.6–2.2 million seniors were immunized at a pharmacy; and 3.3–3.5 million at a clinic. After matching, there were 535,598; 1,017,552; and 1,548,164 in the pharmacy cohort, and 821,662; 1,151,080; and 1,559,488 in the clinic cohort, across study years. The rVE over 2011/12, 2012/13, and 2013/14 during peak influenza circulation was 21.8% (95% CI: −5.9%, 42.3%), 14.8% (9.3%,19.9%), and 16.9% (9.2%, 23.9%), respectively, in the pharmacy cohort; and 16.5% (−5.9%, 34.2%), 15.1% (10.9%, 19.1%), 10.0% (2.9%, 16.6%), respectively, in the clinic cohort. Conclusion HD was consistently associated with better protection against probable influenza events requiring outpatient or inpatient care. The slightly lower treatment effects observed in the outpatient clinic cohort could be a result of confounding by indication due to physicians triaging HD to frailer patients. Disclosures All authors: No reported disclosures.

open forum infectious diseases

2747. Relative Vaccine Efficacy of High-Dose vs. Standard Dose Influenza Vaccines in Preventing Probable Influenza in a US Medicare Fee-for-Service Population