A Study of Atezolizumab With or Without Tiragolumab in Participants With Unresectable Esophageal Squamous Cell Carcinoma Whose Cancers Have Not Progressed Following Definitive Concurrent Chemoradiotherapy

The purpose of this study is to evaluate the efficacy and safety of tiragolumab plus atezolizumab compared with placebo in participants with unresectable esophageal squamous cell carcinoma (or those who are unable or unwilling to undergo surgery) and whose cancers have not progressed following definitive concurrent chemoradiotherapy (dCRT). Participants will be randomized in a 1:1:1 ratio to receive either tiragolumab plus atezolizumab (Arm A), tiragolumab matching placebo plus atezolizumab (Arm B), or double placebo (Arm C).