A Randomized, Double-blind, Placebo-controlled Phase 3 Study to Evaluate Dostarlimab as Sequential Therapy After Chemoradiation in Participants With Locally Advanced Unresected Head and Neck Squamous Cell Carcinoma

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: * Has newly diagnosed unresected Locally Advanced (LA) histologically confirmed HNSCC of the oral cavity, oropharynx, hypopharynx or larynx and completed cisplatin plus radiotherapy (termed "CRT" in this protocol) with curative intent and has no evidence of distant metastatic disease. * Has provided acceptable core or excisional biopsy obtained prior to CRT: * PD-L1 positive tumor status * If the primary tumor site is oropharyngeal carcinoma, the participant must have p…