P-604. Preliminary real-world Abrysvo vaccine effectiveness (VE) against Respiratory Syncytial Virus (RSV)-related lower respiratory tract disease (LRTD) hospitalizations and emergency department (ED) visits—Kaiser Permanente of Southern California (

Abstract Background While the pivotal randomized clinical trial documented the efficacy of the RSVpreF vaccine (Abrysvo) against RSV-related LRTD, the limited number of hospitalizations and ED visits hampered VE assessment for these clinical outcomes. We evaluated Abrysvo’s effectiveness against 1st occurrence of RSV-related inpatient or ED LRTD in KPSC, a large US healthcare system.Table 1.Characteristics of study population, KPSC Methods In this test-negative case control study, we assessed VE among adults ≥60 years of age by analyzing KPSC members’ electronic health records and enhancing specimen testing from 11/24/2023-4/9/2024. Events from inpatient or ED settings meeting an ICD-10 code-based LRTD case definition with a respiratory specimen tested on GenMark RP2 expanded multiplex PCR assay were included. Cases were defined as RSV+ episodes. Analyses were conducted using two sets of pre-specified controls: 1) ‘Strict’: RSV- and positive for a non-vaccine preventable disease (VPD) etiology, and 2) ‘Broad’: all RSV-. The strict approach accounted for lower sensitivity of RSV testing among adults and potential bias associated with VPD controls. Exposure was defined as Abrysvo receipt ≥21 days before encounter. Adjusted odds ratios (OR) and 95% confidence intervals (CI) were estimated from a multivariable logistic regression model with adjustment for patient demographic and clinical characteristics. VE was calculated as 1−OR multiplied by 100%.Table 1.Characteristics of study population, KPSC, cont. Results The overall study population included 4,469 KPSC members: >50% were ≥75 years, 93% had ≥1 comorbidities, and 15% immunocompromised. In the strict analysis, there were 425 cases and 522 controls who received Abrysvo a median of 59 days before illness onset; VE compared to no vaccine receipt was 84% (95% CI: 29–97). In the broad analysis, there were 425 cases and 4,044 controls who received Abrysvo a median of 53 days before illness onset; adjusted VE was 82% (95% CI: 28–96).Table 2.Abrysvo Vaccine Effectiveness against RSV LRTD hospitalizations and ED visits, KPSC Conclusion This study demonstrated real world VE of Abrysvo during its first 5 months of use against severe LRTD in the hospital and ED settings among US adults ≥60 years of age. These results expand on efficacy results from clinical trial populations by including a substantial number of persons age 75+ years and with comorbidities and immunocompromising conditions. Disclosures Sara Y. Tartof, PhD MPH, GSK: Grant/Research Support|Pfizer Inc: Grant/Research Support Negar Aliabadi, MD, MS, Pfizer Inc: employment|Pfizer Inc: Stocks/Bonds (Public Company) Jeff Slezak, MS, Dynavax Technologies: Grant/Research Support|Pfizer: Grant/Research Support Vennis Hong, MPH, Pfizer: Grant/Research Support Bradley Ackerson, MD, Dynavax: Grant/Research Support|GlaxoSmithKline: Grant/Research Support|Moderna: Grant/Research Support|Pfizer: Grant/Research Support Qing Liu, M.S., Pfizer Inc.: Stocks/Bonds (Public Company) Sally Shaw, DrPH, MPH, Pfizer: Grant/Research Support Sabrina Welsh, MPH, Pfizer Inc: Stocks/Bonds (Public Company) Julie Stern, MPH, GlaxoSmithKline: Grant/Research Support|Pfizer: Grant/Research Support|Sanofi: Grant/Research Support Michael Dutro, PharmD, Pfizer: Stocks/Bonds (Public Company) Erica Chilson, PharmD, Pfizer Inc: Employee|Pfizer Inc: Stocks/Bonds (Public Company) Elisa Gonzalez, MS, Pfizer: Stocks/Bonds (Private Company) Robin Hubler, MS, Pfizer, Inc.: Employee of Pfizer Inc.|Pfizer, Inc.: Stocks/Bonds (Private Company) Luis Jodar, PhD, Pfizer Inc: Employment|Pfizer Inc: Stocks/Bonds (Public Company) Bradford D. Gessner, M.D., M.P.H., Pfizer: Employee|Pfizer: Stocks/Bonds (Public Company) Elizabeth Begier, MD, M.P.H., Pfizer Vaccines: Employee|Pfizer Vaccines: Stocks/Bonds (Private Company)