A Study of Atezolizumab Plus Tiragolumab and Atezolizumab Plus Placebo as First-Line Treatment in Participants With Recurrent/Metastatic PD-L1 Positive Squamous Cell Carcinoma of the Head and Neck

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
* Rapidly progressing disease in the opinion of the treating investigator
* 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