The purpose of this study is to learn about the safety and effects of sisunatovir. Sisunatovir is studied for the possible treatment of Respiratory Syncytial Virus (RSV). RSV is a virus that causes lung infections with cold-like symptoms, but it can cause severe illness in some people. Sisunatovir is studied in adults:
* who are not admitted to the hospital and
* who have high chances of having a severe illness from RSV infection.
This study is seeking participants who:
* Are confirmed to have RSV.
* Have symptoms of a lung infection.
* Are 18 years of age or older.
* Have one or more of the following which increases the chances of RSV illness:
* A long-term lung disease.
* heart failure.
* a condition that weakens the immune system.
* Are 65 years of age or older and do not have any of the conditions above
Half of the participants in this study will receive sisunatovir. The other half will receive a placebo for 5 days. Placebo looks same like the study medicine but does not have any medication. Both sisunatovir and placebo will be taken by mouth. The study will compare the experiences of people receiving sisunatovir to those of the people who do not. This will help decide if sisunatovir is safe and effective.
Participants will attend about 8-10 study visits over 5 weeks. During this time, they will have:
* visits at the study clinic,
* blood work,
* swabs of the nose,
* a follow-up phone call.