Inclusion Criteria:

          -  Men and women ≥ 18 years of age.

          -  Provide informed consent signed by study patient or legally acceptable representative.

          -  Willing and able to comply with site visits and study-related procedures and
             requirements.

          -  Histologically confirmed recurrent or metastatic HPV16 positive OPC. Patients with
             squamous cell carcinoma of occult primary site, presenting with lymph node(s) limited
             only to the neck, are also eligible. Patients should have HPV16 positivity confirmed
             before being considered a candidate for this study.

          -  HPV16 positive disease as determined by a specified central reference laboratory
             investigational use only assay.

          -  Patients who have received a minimum of 4 doses of pembrolizumab or nivolumab or
             equivalent anti-PD-1 antibody with or without chemotherapy for 1st or 2nd line
             metastatic HPV16 positive OPC. The last dose of anti-PD-1 must have been no more than
             6 months prior to the first dose of study drug. Progressive disease (PD) must have
             been diagnosed during or after anti-PD-1 therapy (but not longer than 6 months after
             the last dose).

          -  At least one measurable lesion by CT or MRI per RECIST version 1.1 criteria. Target
             lesions may be located in a previously irradiated field if there is documented disease
             progression in that site.

          -  Eastern Cooperative Oncology Group performance status of 0 or 1.

        Exclusion Criteria:

          -  Major surgery within 14 days of first administration of enrollment.

          -  Radiation therapy after progression on prior anti-PD-1 antibody.

          -  Any intervening anti-cancer therapy since last dose of anti-PD-1 including induction
             chemotherapy.

          -  Patients who, after progressing on anti-PD-1, required subsequent chemotherapy within
             the last 4 weeks prior to enrollment in order to control disease.

          -  Patients who have permanently discontinued anti-cancer immune modulating therapies due
             to drug-related toxicity.

          -  Ongoing or recent (within 5 years) evidence of significant autoimmune disease that
             required treatment with systemic immunosuppressive treatments. The following are not
             exclusionary: vitiligo, childhood asthma that has resolved, endocrinopathies (such as
             hypothyroidism or type 1 diabetes) that require only hormone replacement, or psoriasis
             that does not require systemic treatment.

          -  Untreated or active primary brain tumor, central nervous system (CNS) metastases,
             leptomeningeal disease or spinal cord compression.

          -  Encephalitis, meningitis, organic brain disease (e.g. Parkinson's disease) or
             uncontrolled seizures in the year prior to first dose of study therapy.

          -  Known history of, or any evidence of interstitial lung disease, or active,
             non-infectious pneumonitis (in the past 5 years). A history of radiation pneumonitis
             in the radiation field is permitted.