Efficacy and Safety of ALX-0171 in Adult Hematopoietic Stem Cell Transplant (HSCT) Recipients Who Present With Respiratory Syncytial Virus (RSV) Infection

Inclusion Criteria: 1. Subject has received an HSCT using any conditioning regimen and for any underlying etiology (i.e., subject has received an autologous or allogeneic HSCT) 2. Subject is clinically diagnosed with RSV infection with new onset or acute worsening 3. Symptoms likely related to RSV infection have appeared within 5 days of screening and their severity requires initial or maintained hospitalization. 4. Documented RSV infection in the upper respiratory tract (URT) 5. Subject has: * Diagnosis of RSV lower respiratory tract (LRT) disease or * Diagnosis of RSV URT disease with high risk of progression to lower respiratory tract infection (LRTI) Others as defined in the protocol Exclusion Criteria: 1. Subject has clinically significant bacteremia or fungemia within 7 days of screening 2. Subject has clinically significant bacterial, fungal or viral pneumonia 3. Subject presents evidence of shock requiring intensive care unit (ICU) monitoring and/or vasopressor treatment 4. Subject requires or is expected to require invasive mechanical ventilation or intensive non-invasive respiratory support. Standard oxygen supplementation up to 6 L/minute is permitted provided it can be interrupted for the duration of study drug administration. Others as defined in the protocol