A Study of Dostarlimab vs Placebo After Chemoradiation in Adult 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 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 distantmetastatic disease.

• Has provided acceptable core or excisional tissue demonstrating:

• PD-L1 positive tumor status

• If the primary tumor site is oropharyngeal carcinoma, the participant must havep16 IHC testing.

• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

• Has adequate organ function.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Has received prior radiation therapy, systemic therapy, targeted therapy, or radicalsurgery for management of head and neck cancer not considered part of CRT.

• Has cancer outside of the oropharynx, larynx, hypopharynx or oral cavity, such asnasopharyngeal, sinus, other para-nasal, or other unknown primary head and neckcancer.

• Has experienced any of the following with prior immunotherapy: any irAE of Grade ≥3,immune-related severe neurologic events of any grade (e.g., myasthenicsyndrome/myasthenia gravis, encephalitis, Guillain-Barré Syndrome, or transversemyelitis), exfoliative dermatitis of any grade [Stevens-Johnson Syndrome, toxicepidermal necrolysis, or DRESS (Drug Rash with Eosinophilia and Systemic Symptoms)syndrome], or myocarditis of any grade. Non-clinically significant laboratoryabnormalities are not exclusionary.

• Has undergone any major surgical procedure or experienced significant traumaticinjury within 28 days prior to enrolment.

• Has any history of interstitial lung disease or pneumonitis (past or current).

• Has cirrhosis or current unstable liver biliary disease per investigator assessmentdefined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia,esophageal/gastric varices, or persistent jaundice.

• Has a history or current evidence of any medical condition, therapy, or laboratoryabnormality that might confound the study results, interfere with theirparticipation for the full duration of the study intervention, or indicate it is notin the best interest of the participant to participate, in the opinion of theinvestigator.

• Is receiving any other anticancer or experimental therapy. No other experimentaltherapies (including but not limited to chemotherapy, radiation, hormonal treatment,antibody therapy, immunotherapy, gene therapy, vaccine therapy, or otherexperimental drugs) of any kind are permitted while the participant is receivingstudy intervention.

• Previous treatment with anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or an agentdirected to another stimulatory or coinhibitory T-cell receptor [e.g., CytotoxicT-lymphocyte associated protein 4 (CTLA4), OX-40, CD137]

• Is pregnant, breastfeeding, or expecting to conceive children within the projectedduration of the study, starting with the Screening Visit through 120 days after thelast dose of study intervention.

• Has a history of severe allergic and/or anaphylactic reactions to chimeric, human orhumanized antibodies, fusion proteins, or known allergies to dostarlimab or itsexcipients.