Key Inclusion Criteria:

          -  For Japan only, men and women ≥21 years old

          -  Patient with resection of pathologically confirmed CSCC (primary CSCC lesion only, or
             primary CSCC with nodal involvement, or CSCC nodal metastasis with known primary CSCC
             lesion previously treated within the draining lymph node echelon), with macroscopic
             gross resection of all disease

          -  High risk CSCC, as defined in the protocol

          -  Completion of curative intent post-operative radiation therapy (RT) within 2 to 6
             weeks of randomization

          -  Eastern Cooperative Oncology Group performance status (ECOG PS) ≤1

          -  Adequate hepatic, renal, and bone marrow function as defined in the protocol

        Key Exclusion Criteria:

          -  Squamous cell carcinomas (SCCs) arising in non-cutaneous sites as defined in the
             protocol

          -  Concurrent malignancy other than localized CSCC and/or history of malignancy other
             than localized CSCC within 3 years of date of randomization as defined in the protocol

          -  Patients with hematologic malignancies (eg, chronic lymphocytic leukemia (CLL))

          -  Patients with history of distantly metastatic CSCC (visceral or distant nodal), unless
             the disease-free interval is at least 3 years (regional nodal involvement of disease
             in draining lymph node basin that was resected and radiated prior to enrollment will
             not be exclusionary)

          -  Ongoing or recent (within 5 years of randomization date) evidence of significant
             autoimmune disease that required treatment with systemic immunosuppressive treatments,
             which may suggest risk for immune-related adverse events (irAEs). The following are
             not exclusionary: vitiligo, childhood asthma that has resolved, type 1 diabetes,
             residual hypothyroidism that required only hormone replacement, or psoriasis that does
             not require systemic treatment.

          -  Has had prior systemic anti-cancer immunotherapy for CSCC

        Note: Other protocol defined Inclusion/Exclusion criteria apply