Inclusion Criteria:

          -  Capable of giving signed informed consent

          -  Male or female, age >=18 years

          -  Histological or cytological documentation of Head and Neck Squamous Cell Carcinoma
             (HNSCC) that is considered incurable by local therapies

          -  Primary tumor location of the oral cavity, oropharynx, hypopharynx or larynx.

          -  No prior systemic therapy administered in the recurrent or metastatic setting (except
             for systemic therapy given as part of multimodal treatment for locally advanced
             disease)

          -  Measurable disease per RECIST version 1.1 guidelines

          -  ECOG Performance PS score of 0 or 1

          -  Adequate organ function

          -  Life expectancy of at least 12 weeks

          -  Female participants: must not be pregnant, not breastfeeding, and at least one of the
             following conditions apply:

               1. Not a woman of childbearing potential (WOCBP)

               2. A WOCBP who agrees to use a method of birth control from 30 days prior to
                  randomization and for at least 120 days after the last dose of study treatment.

          -  Male participants with female partners of child-bearing potential: must agree to use a
             highly effective contraception while receiving study treatment and for at least 120
             days after the last dose of study treatment and refrain from donating sperm during
             this period.

          -  Provide tumor tissue from excisional or core biopsy (fine needle aspirates and bone
             biopsies are not acceptable) acquired within 2 years prior to randomization for PD-L1
             immunohistochemistry (IHC) testing by central laboratory.

          -  Have PD-L1 IHC CPS 1 status by central laboratory testing

          -  Have results from testing of Human Papilloma Virus (HPV) status for oropharyngeal
             cancer

        Exclusion Criteria:

          -  Prior therapy with an anti-PD-1/L1/L2 and/or anti-ICOS directed agent

          -  Systemic approved or investigational anticancer therapy within 30 days or 5 half-lives
             of the drug, whichever is shorter.

          -  Major surgery 28 days prior to randomization.

          -  Toxicity from previous anticancer treatment that includes toxicity related to prior
             treatment that has not resolved to Grade 1 (except alopecia, hearing loss,
             endocrinopathy managed with replacement therapy, and peripheral neuropathy which must
             be Grade 2)

          -  Received transfusion of blood products or administration of colony stimulating factors
             within 14 days prior to randomization

          -  Central nervous system (CNS) metastases, with the following exception: Participants
             with asymptomatic CNS metastases who are clinically stable and have no requirement for
             steroids for at least 14 days prior to randomization

          -  Invasive malignancy or history of invasive malignancy other than disease under study
             within the last 3 years, except as noted below:

             a. Any other invasive malignancy for which the participant was definitively treated,
             has been disease-free for 3 years and in the opinion of the principal investigator and
             GSK Medical Monitor will not affect the evaluation of the effects of the study
             treatment on the currently targeted malignancy, may be included in this clinical study

          -  Autoimmune disease or syndrome that required systemic treatment within the past 2
             years

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroids (≥10 mg oral
             prednisone per day or equivalent) or other immunosuppressive agents within 7 days
             prior to randomization

          -  Receipt of any live vaccine within 30 days prior randomization

          -  Prior allogeneic/autologous bone marrow or solid organ transplantation

          -  Has current pneumonitis or history of non-infectious pneumonitis that required
             steroids or other immunosuppressive agents

          -  Recent history (within the past 6 months) of uncontrolled symptomatic ascites, pleural
             or pericardial effusions

          -  Recent history (within the past 6 months) of gastrointestinal obstruction that
             required surgery, acute diverticulitis, inflammatory bowel disease, or intra-abdominal
             abscess

          -  Recent history of allergen desensitization therapy within 4 weeks of randomization

          -  History or evidence of cardiac abnormalities within the 6 months prior to
             randomization.

          -  Cirrhosis or current unstable liver or biliary disease per investigator assessment
             defined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia,
             esophageal or gastric varices, or persistent jaundice.

          -  Active infection requiring systemic therapy

          -  Known HIV infection, or positive test for hepatitis B active infection (presence of
             hepatitis B surface antigen), or hepatitis C active infection

          -  History of severe hypersensitivity to monoclonal antibodies or any ingredient used in
             the study treatment formulations

          -  Known history of active tuberculosis

          -  Any serious and/or unstable pre-existing medical (aside from malignancy), psychiatric
             disorder, or other condition that could interfere with participant's safety, obtaining
             informed consent, or compliance to the study procedures in the opinion of the
             investigator

          -  Is currently participating in (unless in follow-up phase and 4 weeks have elapsed from
             last dose of prior investigational agent), or has participated in a study of an
             investigational agent or has used an investigational device within 4 weeks prior to
             date of randomization