Inclusion Criteria:  Histologically confirmed diagnosis of a non-resectable recurrent or metastatic head and neck squamous cell carcinoma (not necessarily reconfirmed at time of enrolment) At least one measurable tumor lesion as per RECIST v1.1, (Scan not older than 4 weeks before randomization) Eligible for pembrolizumab monotherapy (PD-L1 CPS >/= 1% and adequate laboratory parameters for pembrolizumab monotherapy as assessed by the investigator) ECOG-performance score 0-2 Written informed consent obtained according to international guidelines and local laws Ability to understand and give informed consent. Safe contraception measures for males and females. Procedures with a pearl index of less than 1% apply as safe pregnancy prevention measures.  Exclusion Criteria:  Patients for whom a combination therapy of a checkpoint inhibitor and a chemotherapy is deemed necessary in the opinion of the investigator Participation in another interventional study simultaneously and within the last 30 days prior to inclusion (registries or observational studies allowed) Concurrent malignancies other than disease under study within 5 years prior to inclusion, with the exception of those with a negligible risk of metastasis or death (e.g., expected 5-year OS > 90%) treated with expected curative outcome Active, known, or suspected autoimmune disease requiring systemic treatment. A concomitant therapy with systemic immune suppression: use of chronic systemic steroid medication (up to 5 mg/day prednisolone equivalent is allowed; patients using physiological replacement doses of prednisone for adrenal or pituitary insufficiency are eligible) History of severe autoimmune disorder or history of organ transplant Any serious or uncontrolled medical disorder or active infection that, in the opinion of the investigator, may increase the risk associated with study participation, study drug administration, or would impair the ability of the subject to receive study drug. Significant acute or chronic infections including, among others (test not older than 4 weeks prior to randomization): Any positive test for human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome (AIDS), Any positive test result for hepatitis B virus or hepatitis C virus indicating acute or chronic infection. Pregnancy or lactation (Bacterial) infections requiring systemic antibiotic treatment within 2 weeks prior to first dose of study treatment (depending on group assignment: either prior to first UV1 or prior to first pembrolizumab administration). History of allergy or hypersensitivity to study drug or human granulocyte-macrophage colony stimulating factor, yeast-derived products or any constituent of the products Receipt of a live vaccine within 30 days prior to start of therapy Patient who has been incarcerated or involuntarily institutionalized by court order or by the authorities. Patients who are unable to consent because they do not understand the nature, significance and implications of the clinical study and therefore cannot form a rational intention in the light of the facts.