Study of Carboplatin Plus Etoposide With or Without SHR-1316 in Participants With Untreated Extensive-Stage (ES) Small Cell Lung Cancer (SCLC)

Inclusion Criteria: * Age \>= 18 years and \<= 75 years * Histologically or cytologically confirmed ES-SCLC (per the Veterans Administration Lung Study Group \[VALG\] staging system) * Eastern Cooperative Oncology Group performance status of 0 or 1 * No prior systemic treatment or immune checkpoint inhibitor treatment for ES-SCLC * At least 6 months treatment-free period since last chemo/radiotherapy with curative intent for limited-stage SCLC * Measurable disease, as defined by RECIST v1.1 * Adequate hematologic and end organ function * Patients must submit a pre-treatment tumor tissue sample during the study. * Signed inform consent form Exclusion Criteria:• Active or untreated central nervous system (CNS) metastases as determined by computed tomography (CT) or magnetic resonance imaging (MRI) evaluation * Spinal cord compression not definitively treated with surgery and/or radiation or previously diagnosed and treated spinal cord compression without evidence that disease has been clinically stable for ≥ 1 week prior to randomization * Leptomeningeal disease * Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures * Uncontrolled or symptomatic hypercalcemia * Malignancies other than SCLC within 5 years prior to randomization, with the exception of those with a negligible risk of metastasis or death treated with expected curative outcome * History of autoimmune disease, including but not limited to myasthenia gravis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease * Prior treatment with immune checkpoint blockade therapies * Subjects with a condition requiring systemic treatment with either corticosteroids (\>10 mg daily prednisone equivalent) or other immunosuppressive medications within 14 days of first administration of study treatment. Inhaled or topical steroids, and adrenal replacement steroid are permitted in the absence of active autoimmune disease. * Significant cardiovascular disease * Prior allogeneic bone marrow transplantation or solid organ transplant * Treatment with systemic immunosuppressive medications within 2 weeks prior to randomization * History of hypersensitivity reactions to chimeric or humanized antibodies or fusion proteins, carboplatin or etoposide * History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the treating investigator