Head and neck cancer is the malignant tumor with the highest morbidity and mortality, of which 60% present with locally advanced disease at initial diagnosis, and the 5-year survival rate of standard treatment is less than 30%. Standard of care (SOC) including adjuvant and neoadjuvant therapy can provides only about 5-10% clinical benefit. According to the available data on the application of immunotherapy as adjuvant therapy in operable patients, adjuvant immunotherapy is safe and feasible, with a significant trend of benefit. Based on the above positive and meaningful clinical needs and scientific basis, it is very necessary to carry out clinical trials of adjuvant immunotherapy. The primary objective of this study is to evaluate the efficacy and safety of immune maintenance therapy in patients with locally advanced head and neck squamous cell carcinoma who achieve 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.