Researchers are looking for other ways to prevent severe illness from COVID-19. COVID-19 is a virus that most often causes mild flu or cold-like symptoms. However, people with certain health conditions or other factors have a high risk (chance) of getting severely ill from COVID-19, which can require a hospital stay or lead to death. Some people who are high risk for severe illness may be unable to take certain treatments for COVID-19 because they are not available to them, or they take other medicines that may react with a treatment and cause an unwanted effect. Molnupiravir (MK-4482) is a study medicine designed to stop the COVID-19 virus from copying itself in the body (multiplying). The goal of this study is to learn if molnupiravir prevents severe illness from COVID-19 more than placebo in people who are high risk.