898. Influenza Vaccination Reduces Risk of Severe Outcomes among Adults Hospitalized with Influenza A(H1N1)pdm09, FluSurv-NET, 2013–2018

BACKGROUND: Influenza vaccination may reduce illness severity among those with influenza; however, data are limited. We determined whether outcomes were less severe among vaccinated compared with unvaccinated adults hospitalized with influenza over 5 seasons. METHODS: We included adults (≥18 years) hospitalized with laboratory-confirmed influenza during seasons 2013–2014 through 2017–2018 and identified through the US Influenza Hospitalization Surveillance Network (FluSurv-NET). Vaccination status was obtained through medical records, vaccine registries, and interviews. We excluded patients who were institutionalized, did not receive antivirals, or had unknown vaccine status or vaccine receipt <14 days before positive influenza test. We used inverse propensity score weighting to balance differences between vaccinated and unvaccinated groups and multivariable logistic and competing risk regression to evaluate the association between vaccination and outcomes including pneumonia, intensive care unit (ICU) admission, mechanical ventilation (MV), death, and ICU and hospital length of stay (LOS) in days. Models were adjusted for season and admission timing in relation to timing of antiviral treatment, symptom onset and season peak. RESULTS: Among 67,452 adults hospitalized with influenza, 43,608 were included; 47% were 18–64 years (38% vaccinated) and 53% were ≥65 years (65% vaccinated). Among patients with influenza A(H1N1)pdm09, vaccination was associated with decreased odds of ICU admission (odds ratio (OR) 0.81; OR 0.72) and MV (OR 0.66; OR 0.54) in adults 18–64 and ≥65 years, respectively; decreased odds of pneumonia (OR 0.83), death (OR 0.64) and shortened ICU LOS (relative hazard (RH) 0.82) in adults 18–64 years; and shortened hospital LOS (RH 0.91) in adults ≥65 years (figure). Vaccination was not associated with attenuation of severe outcomes in patients with influenza A(H3N2) and B. CONCLUSION: Vaccination was associated with reduced odds of severe outcomes, including death, by up to 36% in adults hospitalized with influenza A(H1N1)pdm09. All adults without contraindications should receive annual influenza vaccination as there is evidence that it can improve outcomes among those who develop influenza despite vaccination. [Image: see text] DISCLOSURES: All Authors: No reported Disclosures.