A Study of Atezolizumab (Anti-PD-L1 Antibody) as Adjuvant Therapy After Definitive Local Therapy in Patients With High-Risk Locally Advanced Squamous Cell Carcinoma of the Head and Neck

Inclusion Criteria

* Histologically or cytologically confirmed Squamous Cell Carcinoma of the Head and Neck (SCCHN)
* Human Papilloma Virus (HPV) status
* Completed definitive local therapy
* Absence of metastatic disease as documented by radiographic scans
* Adequate hematologic and end-organ function
* For patients receiving therapeutic anticoagulation: stable anticoagulant regimen
* For women of childbearing potential: agreement to remain abstinent or use contraceptive methods with a failure rate of \< 1% per year during the treatment period and for 5 months after the last dose of study treatment. Women must refrain from donating eggs during this same period.
* Confirmed response of Complete Response (CR), Partial Response (PR), or Stable Disease (SD) to definitive local therapy documented by CT with contrast or MRI with contract to head and neck region done \>= 8 weeks after completion of definitive local therapy and within 28 days prior to initiation of study drug.

Exclusion Criteria:

* Patients who have received surgery alone or radiotherapy alone as definitive local therapy
* Squamous cell carcinoma of the nasopharynx or paranasal sinuses or non-squamous histology
* Evidence of disease progression or metastatic disease during or following definitive local therapy documented in post-definitive local therapy screening scans
* Uncontrolled or symptomatic hypercalcemia
* Active or history of autoimmune disease or immune deficiency
* Active tuberculosis
* Significant cardiovascular disease
* History of malignancy, including prior SCCHN primary tumors within 5 years prior to screening, with the exception of malignancies with a negligible risk of metastasis or death
* Prior allogeneic stem cell or solid organ transplantation
* Current treatment with anti-viral therapy for Hepatitis B Virus (HBV)
* Treatment with systemic immunostimulatory agents
* Treatment with systemic immunosuppressive medication
* History of severe allergic anaphylactic reactions to chimeric or humanized antibodies or fusion proteins
* Pregnancy or breastfeeding, or intention of becoming pregnant during study treatment or within 5 months after the last dose of study treatment
* Patients who have received a non-FDA or non-EMA approved anti-EGFR agent or any other non-FDA or non-EMA, approved agent as part of definitive local therapy, unless the unapproved agent was given in addition to an approved agent
* Any systemic therapies after permitted definitive local therapies