A Study of 2 Treatment Plans of Nirmatrelvir With Ritonavir Compared to Placebo in Preventing COVID-19 Among Adults Who Lived in the Same House With a Person Who Had COVID-19. A Phase 2/3, Randomized, Double-Blind, Double-Dummy, Placebo-Controlled Study to Evaluate the Safety and Efficacy of 2 Regimens of Orally Administered PF-07321332/Ritonavir in Preventing Symptomatic SARS-CoV-2 Infection in Adult Household Contacts of an Individual With Symptomatic COVID-19.