A Study to Learn About the Effects of Multiple Doses of Sisunatovir on People With Respiratory Syncytial Virus (RSV) Infection

The purpose of the study is to learn about the safety, pharmacokinetics and antiviral activity of the study medicine (called sisunatovir/RV521) for the potential treatment of respiratory syncytial virus (RSV). Sisunatovir will be given as multiple doses during the treatment period. RSV is a highly contagious virus that can lead to serious lung infections in patients with reduced ability to fight infection. Most vulnerable populations include babies, the elderly and patients that have received a bone marrow transplant.

This study is seeking healthy participants who are:

1. Aged 18 to 45 years old and will agree to the use of highly effective methods of contraception. 2. with a body mass index (BMI) of 18.0 to 30.0 Kg/m2 This study will consist of 2 cohorts of 33 participants each. In both cohorts participants will be exposed to the challenge virus on study day 0. Cohort 1 will receive either 200 mg of sisunatovir or placebo (looks the same as sisunatovir but contains no active medicine) 2 times a day for 5 days. Cohort 2 will receive either 350 mg of sisunatovir or placebo 2 times a day for 5 days. Participants will start taking the study medicine upon confirmation of RSV infection (or evening of Day 5 if not positive to RSV). The study medicine will be administered 12 hours apart (or twice daily). Each participant will remain in the quarantine unit until discharge on Day 12.