The purpose of the study is to assess if the study medication (molnupiravir, MK-4482) will prevent symptomatic coronavirus disease 2019 (COVID-19) in adults who live with someone with confirmed COVID-19 infection. This is a phase 3, multicenter, randomized, double-blind, placebo-controlled study; half of the study participants will receive molnupiravir twice daily by mouth and the other half will receive a placebo. The primary objectives of the study are to determine if molnupiravir prevents symptomatic COVID-19 disease and to evaluate its safety and tolerability. All participants who develop COVID-19 during the study are still eligible for any COVID-19 treatment recommended by their doctor.