A Study to Learn About the Effects of Sisunatovir in Infants With Respiratory Syncytial Virus Lower Respiratory Tract Infection.

This is a multicentre, 3-part study to evaluate safety, tolerability, PK, PD, and antiviral effect of single and multiple dosing of RV521 in infants hospitalised due to RSV LRTI. The clinical study consists of 3 parts, the third part (Part C) is optional: * Part A is an open-label, multicentre, single dose study in infants hospitalised with RSV LRTI (Cohorts 1 \& 2) * Part B is a randomised, double-blind, placebo-controlled, multicentre multiple dose study in infants hospitalised with RSV LRTI (Cohorts 3, 4 \& 5) * Part C is a randomised 1:1, double-blind, placebo-controlled, multicentre, multiple-dose study in infants hospitalised with RSV LRTI I The number of subjects enrolled in Parts A and B of the study will depend on the safety and PK data from the group of subjects enrolled in specified age cohorts and the subsequent recommendation of the Data Safety Monitoring Committee (DSMC). The DSMC may recommend a dose adjustment (either a reduction or an escalation) and/or regimen adjustment (Part B only) for subsequent subjects because of the observation of an unexpected safety/tolerability profile and/or differences between the observed and predicted exposure resulting from a specified dose of RV521.