Key Inclusion Criteria:

          -  Histologically or cytologically confirmed recurrent/metastatic SCCHN involving the
             oropharynx, oral cavity, larynx, or hypopharynx, that is considered incurable by local
             therapies

          -  Known results from human papillomavirus (HPV) status test for oropharyngeal carcinoma

          -  No prior systemic therapy for metastatic and/or recurrent SCCHN

          -  Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1

          -  Tumor PD-L1 expression as determined by PD-L1 immunohistochemistry assay

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

          -  Life expectancy >=12 weeks

        Key Exclusion Criteria:

          -  Disease suitable for local therapy with curative intent

          -  Progressive or recurrent disease within 6 months of the last dose of curative intent
             systemic treatment for locally advanced SCCHN

          -  Grade >=2 unresolved toxicity related to surgery or other prior therapies

          -  Symptomatic, untreated, or actively progressing central nervous system (CNS)
             metastases

          -  History of leptomeningeal disease

          -  Active or history of autoimmune disease or immune deficiency

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced
             pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening
             chest computed tomography (CT) scan

          -  History of additional malignancy other than SCCHN within 5 years prior to
             randomization

          -  Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including
             anti-CTLA-4, anti-TIGIT, anti-PD-L1, and anti-PD-1 therapeutic antibodies

          -  Treatment with systemic immunostimulatory agents or systemic immunosuppressive
             medication

          -  Pregnancy or breastfeeding