Sintilimab or Placebo With Chemotherapy in Esophageal Squamous Cell Carcinoma ( ORIENT-15 )

Key Inclusion Criteria: * Histopathologically confirmed unresectable, locally advanced, recurrent or metastatic ESCC (excluding mixed adenosquamous carcinoma and other histological subtypes) * ECOG PS of 0 or 1 * Subject must be unsuitable for definitive treatment, such as definitive chemoradiotherapy and/or surgery. For subjects who have received (neo)adjuvant or definitive chemotherapy/radiochemotherapy, time from the completion of last treatment to disease recurrence must be \> 6 months Could provide archival or fresh tissues for PD-L1 expression analysis with obtainable results * Have at least one measurable lesion as per RECIST v1.1 Key exclusion Criteria: * ESCC with endoscopy-confirmed near-complete obstruction requiring interventional therapy * Post stent implantation in the esophagus or trachea with risk of perforation * Received systemic treatment for advanced or metastatic ESCC. * Received a cumulative dose of cisplatin ≥ 300 mg/m2 and the last cisplatin dose was within 12 months of randomization or the first dose of study treatment in the open-label phase. * High risk of hemorrhage or perforations due to tumor invasion in adjacent organs (aorta or trachea), or have fistula formation. * Hepatic metastasis \> 50% of the total liver volume. * Received palliative therapy for a local lesion within 2 weeks prior to the first dose. * Received systemic treatment with Chinese traditional medicines with anti-cancer indications or immunomodulators (including thymosins, interferons, and interleukins) within 2 weeks prior to the first dose of study treatment. * Received systemic immunosuppressants within 2 weeks prior to randomization, excluding local use of glucocorticoids administered by nasal, inhaled, or other routes, and systemic glucocorticoids at physiological doses (no more than 10 mg/day of prednisone or equivalents), or glucocorticoids to prevent allergies to contrast media.