- We developed an Immunodeficiency Scoring Index for Respiratory Syncytial Virus (ISI-RSV) infection, based on a cohort of 237 allogeneic hematopoietic cell transplant (allo-HCT) recipients, that can predict the risk of progression to lower respiratory tract infection (LRTI) and RSV-associated mortality. A weighted index was calculated using adjusted hazard ratios for immunodeficiency markers including age> 40, absolute neutrophils and lymphocytes counts. Based on the ISI-RSV (range: 0-12), patients were stratified into low (0-2), moderate (3-6), and high (7-12) risk groups. A significant trend of increasing incidence of progression to LRTI and RSV-associated mortality was observed as the risk increased from low to moderate to high (p < .001). Patients in the high-risk group had the greatest benefit of ribavirin-based therapy at the upper respiratory tract infection stage and the highest risk for progression to LRTI and death when antiviral therapy was not given [6.5 (95% CI, 1.8-23.6) and 8.1 (95% CI, 1.1-57.6), respectively]. The ISI-RSV is designed to stratify allo-HCT recipients with RSV infection into groups according to their risk for progression to LRTI and RSV-associated mortality. Identification of high-risk groups using this index would distinguish patients who would benefit the most from antiviral therapy, mainly with aerosolized ribavirin. The ISI-RSV should be validated in a multi-institutional study.