A Multicentre Study in Otherwise Healthy Infants and Toddlers Hospitalised For and Diagnosed With RSV Lower Respiratory Tract Infection to Evaluate the Safety, Tolerability, and Clinical Activity of ALX-0171

Inclusion Criteria: 1. Subject is otherwise healthy, but hospitalised for and clinically diagnosed with RSV LRTI (Lower Respiratory Tract Infection) 2. Subject has appearance of upper or lower respiratory tract infection symptoms that are likely related to RSV 3. Subject has a positive RSV diagnostic test 4. Others as defined in the protocol Exclusion Criteria: 1. Subject has history of wheezing 2. Subject is known to have significant comorbidities 3. Subject is known to be immunocompromised 4. Subject is suspected of having a clinically relevant infection other than RSV 5. Others as defined in the protocol