A Study of Dostarlimab vs Placebo After Chemoradiation in Adult Participants With Locally Advanced Unresected Head and Neck Squamous Cell Carcinoma

INTERVENTION: Intervention1: Dostarlimab: 50 mg/mL with a delivery volume of 10 mL 500 mg Q3W for 4 cycles then, 1000 mg Q6W for 7 cycles Control Intervention1: Placebo : Commercially sourced sterile 0.9% Sodium Chloride solution for intravenous infusion (normal saline) Q3W for 4 cycles then Q6W for 7 cycles CONDITION: Health Condition 1: C148‐ Malignant neoplasm of overlappingsites of lip, oral cavity and pharyn X PRIMARY OUTCOME: Event‐free Survival (EFS) Assessed by Blinded Independent Central Review (BICR)Timepoint: Up to approximately 5 years SECONDARY OUTCOME: Event‐free Survival (EFS) assessed by investigatorTimepoint: Up to approximately 5 years Number of Participants with Anti‐Drug Antibodies against DostarlimabTimepoint: Up to approximately 15 months Number of participants with clinically significant changes in laboratory, vital signs, and safety assessment parametersTimepoint: Up to approximately 5 years Number of Participants with TEAEs and SAEs leading to dose delays, withdrawals or deathTimepoint: Up to approximately 5 years Number of Participants with treatment emergent adverse events (TEAEs), Immune‐mediated TEAEs, and serious adverse events (SAEs) by severityTimepoint: Up to approximately 5 years Overall Survival (OS)Timepoint: Up to approximately 5 years Serum Concentration of Dostarlimab at End of Infusion (C‐EoI)Timepoint: Up to approximately 15 months Serum Concentration of DostarlimabTimepoint: Up to approximately 15 months Serum Predose trough concentration (Ctrough) of DostarlimabTimepoint: Up to approximately 15 months INCLUSION CRITERIA: Participants are eligible to be included in the study only if all of the following criteria apply: 1. Has newly diagnosed unresected LA histologically confirmed HNSCC of the oral cavity, oropharynx, hypopharyn Xor laryn Xand completed cisplatin plus radiotherapy (termed "CRT" in this protocol) with curative intent and has no evidence of distant metastatic disease. 2. Has provided acceptable core or excisional tissue demonstrating: PD‐L1 positive tumor status If the primary tumor site is oropharyngeal carcinoma, the participant must have p16 IHC testing. 3. Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 4. Has adequate organ function.