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Background Despite the high disease burden of RSV in older adults and children, there is currently no approved vaccine. Ad26.RSV.preF, an experimental RSV vaccine, has demonstrated immunogenicity and tolerability in first-in-human clinical studies. The aim of this study was to assess the potential of the Ad26.RSV.preF vaccine to protect against RSV infection and disease in an established RSV human challenge model, used for the first time to evaluate a vaccine. Methods We conducted a randomized, double-blind, placebo-controlled, human challenge study (NCT03334695). Healthy adults received 1 × 1011 vp Ad26.RSV.preF vaccine (active) or placebo (pbo) intramuscularly. After 28 days, volunteers were challenged intranasally with a low-passage clinical strain of RSV-A (0.8 mL of Memphis 37b) and then quarantined for 12 days. Nasal washes were collected twice daily throughout quarantine, starting 2 days post-challenge (viral load [VL] by qRT-PCR and quantitative cultures). Disease severity was recorded thrice daily using symptom diary cards. Results Fifty-three volunteers (active, n = 27; pbo, n = 26) were challenged with RSV-A. Quantitative viral assessments were consistently lower in active than pbo. The primary endpoint of the study was met: the area under the curve (AUC) for RSV VL over time (via qRT-PCR) was significantly lower in active pbo (P = 0.012). Median peak VL was lower for active (0 log10 copies/mL) than pbo (5.4 log10 copies/mL). Median AUC for RSV VL over time (quantitative culture) was lower for active than pbo (0 vs. 109, P = 0.002). Disease severity was lower for active than pbo, with a median AUC total symptom score of 35 (active) vs. 167 (pbo) (P = 0.002). Overall, RSV infection (defined by qRT-PCR alone or combined with symptoms) and disease severity over time were lower in active vs. pbo. Conclusion RSV infections, VL, and RSV disease severity were consistently lower in healthy adults receiving Ad26.RSV.preF vs. placebo, demonstrating promising protection from RSV infection and disease. This was the first time that antiviral prevention was observed against RSV after active immunization. Ad26.RSV.preF warrants further evaluation in field trials for efficacy against natural RSV infections in populations considered at risk of severe RSV disease. Disclosures All Authors: No reported Disclosures.

open forum infectious diseases

902. A Randomized, Double-Blind, Placebo-Controlled Study to Evaluate the Efficacy of a Single Immunization of Ad26.RSV.preF against RSV Infection in a Viral Challenge Model in Healthy Adults