A Single-center Phase II Study to Investigate the Immune Maintenance Therapy Pattern in Patients With Locally Advanced Head and Neck Squamous Cell Carcinoma Who Have Achieved MPR After Neoadjuvant Immunotherapy Combined With Chemotherapy

Inclusion Criteria: * Able to understand and willing to give an informed consent for the study. * Males or females aged 18 to 80 years. * Have an Eastern Co-operative Oncology Group (ECOG) performance status less than equal to 1. * Pathologically (histologically or cytologically) confirmed, non-metastatic diagnosis of squamous cell carcinoma of head and neck (SCCHN). * Achievement of major pathological reaponse (MPR) after surgery with neoadjuvant immunotherapy combined chemotherapy. * Adequate bone marrow, liver, and renal function: Absolute neutrophil count ≥ 1.5 × 10\^9/L, hemoglobin ≥ 9.0g/dL, platelets ≥100000/μL; ALT and AST \< 2.5× upper limit of normal (ULN), total bilirubin ≤ 1.5×ULN； Creatinine clearance ≥ 60 ml/min； APTT≤ 1.5×ULN. Exclusion Criteria: * Participant has metastatic/unresectable SCCHN. * Have an active autoimmune disease requiring systemic treatment or a documented history of clinically severe autoimmune disease. * With active hepatitis B or C, or known history of positive HIV test, or acquired immunodeficiency syndrome. * With interstitial pneumonitis, non-infectious pneumonitis, active pulmonary tuberculosis, or history of pulmonary tuberculosis infection that were not controlled by treatment. * With active infection requiring systemic therapy. * Received any investigational drug within 4 weeks prior to the first dose, or concurrently enrolled in another clinical trial. * Any other factors that are not suitable for inclusion in this study judged by investigators.