1. Participants will have completed standard-of-care definitive cCRT with curative intent

within 12 weeks (84 days) prior to randomization as presented below. If baseline CT/MRI

images are equivocal and/or residual disease is suspected, investigators are strongly

recommended to perform an FDG-PET assessment to confirm absence of progression.

Where FDG-PET-avid lesions are detected and accessible, an endoscopy is recommended

to be performed and a biopsy taken as clinically indicated to confirm residual disease and

evaluate whether participation in this study is appropriate.

1. Participants must not have progressed following definitive cCRT therapy with curative

intent. Absence of recurrence and/or progression will be assessed by the following

imaging procedures up to 12 weeks after the last dose of cCRT.

1. Local disease (primary tumor site and neck) will be assessed by CT (preferred) or

MRI with contrast from mid-orbits to thoracic inlet.

1. Absence of disease outside of the head and neck will be assessed by CT with contrast

(preferred) of the chest, abdomen including liver, and pelvis as clinically indicated or

by MRI if CT with IV contrast is contraindicated.

1. Confirmation of PD-L1 expression from tumor tissue sample per Inclusion Criterion 4

(all participants) by the central laboratory. Participants with unknown PD-L1 expression

prior to randomization are not eligible for the study

1. Adequate organ and bone marrow function (in the absence of transfusions or growth

factor support within 14 days prior to Screening Part II).