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
Have histologically confirmed (by local pathology) R/M HNSCC that is considered incurable by local therapies
Primary tumor location of oral cavity, oropharynx, larynx, or hypopharynx (patients with cervical neck node SCC with occult primary as described in the protocol
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
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
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
Participants who have a primary tumor site of nasopharynx, paranasal sinus or salivary gland (any histology)
Head and neck SCC with unknown primary site as described in the protocol
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