RecruitingPhase 2Drug trialA Study of Pembrolizumab in Combination With Chemotherapy for Head and Neck CancerBy doing this study, the research team would like to learn if using a blood test that measures the amount of tumor DNA in blood can help guide how to use chemotherapy combined with immunotherapy for individuals with head and neck cancer. Using this blood test, the research team hopes to learn if intermittent (occasional) chemotherapy added to immunotherapy will work better than immunotherapy alone. Participation in this research will last about two years.