Inclusion Criteria:
1. Provide written informed consent and be willing and able to comply with study requirements and scheduled assessments;
2. Be aged 18-75 years at the time of consent;
3. Have an ECOG performance status of 0 or 1;
4. Have histologically or cytologically confirmed small-cell lung cancer;
5. Have radiologically confirmed limited-stage disease;
6. Have achieved a complete response, partial response, or stable disease after concurrent or sequential chemoradiotherapy, according to RECIST version 1.1;
7. Have a life expectancy of at least 3 months;
8. Have adequate organ function.
Exclusion Criteria:
1. Received systemic immunostimulatory agents, including interferons, interleukin 2, or tumour necrosis factor, within 4 weeks or five half-lives before the first dose of study treatment, whichever is longer; previous cancer vaccines are permitted;
2. Received any traditional Chinese herbal medicine for cancer control within 14 days before the first dose;
3. Required systemic corticosteroids equivalent to more than 10 mg per day of prednisone or other immunosuppressive treatment within 14 days before the first dose;
4. Received a live vaccine within 4 weeks before the first dose; inactivated seasonal influenza vaccines are permitted, but live intranasal influenza vaccines are not;
5. Underwent major surgery requiring general anaesthesia within 28 days before the first dose;
6. Had previous allogeneic stem-cell or organ transplantation;
7. Have clinically significant pericardial effusion;
8. Have uncontrolled pleural effusion or ascites requiring drainage within 2 weeks before enrolment;
9. Have active autoimmune disease or a history of autoimmune disease with a risk of recurrence;
10. Have a history of interstitial lung disease or non-infectious pneumonitis, or uncontrolled systemic disease, including diabetes, hypertension, pulmonary fibrosis, or acute lung disease;
11. Have a severe chronic or active infection requiring systemic antibacterial, antifungal, or antiviral treatment within 2 weeks before the first dose, including tuberculosis;
12. Have had another active malignancy within 2 years before the first dose, except the cancer under investigation or definitively treated localised cancers, including resected basal-cell or squamous-cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix or breast;
13. Have untreated chronic hepatitis B, chronic HBV infection with an HBV DNA concentration of at least 500 IU/mL (2500 copies/mL), or active hepatitis C;
14. Have a known history of HIV infection;
15. Have clinically significant cardiovascular risk factors;
16. Have unresolved toxicities from previous anticancer treatment that have not returned to baseline or stabilised, except adverse events unlikely to pose a safety risk, such as alopecia, neuropathy, or specified laboratory abnormalities;
17. Have a history of severe hypersensitivity to monoclonal antibodies.
