This study is researching an experimental drug called fianlimab (also called REGN3767), combined with a medication called cemiplimab compared against cemiplimab combined with placebo (a placebo looks like a treatment but does not contain any real medicine), collectively called "study drugs" in this form.

The study is focused on participants with head and neck cancers who have not been previously treated for head and neck cancer that has come back or spread to other parts of the body, referred to as recurrent or metastatic (R/M) head and neck squamous cell carcinoma (HNSCC).

The study is looking at several other research questions, including:

• What side effects may happen from taking the study drugs
• How much of each study drug is in the blood at different times
• Whether the body makes antibodies against the study drug(s) individually (which could make the study drugs less effective or could lead to side effects)
• Compatible research to better understand the study drugs and HNSCC

Key Inclusion Criteria:

1. Have histologically confirmed (by local pathology) R/M HNSCC that is considered incurable by local therapies
2. Primary tumor location of oral cavity, oropharynx, larynx, or hypopharynx (patients with cervical neck node SCC with occult primary as described in the protocol
3. PD-L1 expression Combined Positive Score (CPS) ≥1 documented with a previously PD-L1 obtained Immunohistochemistry (IHC) result prior to screening, as described in protocol
4. Oropharynx cancer participants only: HPV status, based on a previously documented result prior to screening, must have been established in a surgical biopsy specimen or a core biopsy specimen as described in the protocol
5. At least 1 lesion that is measurable by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 as described in the protocol
6. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
7. Adequate organ and bone marrow function as described in the protocol

Key Exclusion Criteria:

Medical Conditions

1. Participants who have Progressive Disease (PD) within 6 months of completion of curatively intended systemic treatment for locoregionally advanced HNSCC as described in the protocol

2. Participants who have a primary tumor site of nasopharynx, paranasal sinus or salivary gland (any histology)

3. Head and neck SCC with unknown primary site as described in the protocol

4. Participants with active, known, or suspected autoimmune disease that has required systemic therapy within 5 years of the projected enrollment date as described in the protocol

5. History of interstitial lung disease (eg, idiopathic pulmonary fibrosis, organizing pneumonia) or active, noninfectious pneumonitis that required immune-suppressive doses of glucocorticoids to assist with management

6. History or current evidence of significant cardiovascular disease including, myocarditis, congestive heart failure (as defined by New York Heart Association Functional Classification III and IV), unstable angina, serious uncontrolled arrhythmia, and myocardial infarction 6 months prior to study enrollment.

   Prior/Concomitant Therapy

7. Participants who have received prior systemic anticancer therapy in the R/M HNSCC setting as described in the protocol

8. Participants with a condition requiring corticosteroid therapy (>10 mg prednisone/prednisolone/day or equivalent) within 14 days of the first dose of study drug as described in the protocol

Note: Other protocol defined Inclusion/ Exclusion Criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy.

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf