Inclusion Criteria for the Induction Phase:  ECOG PS of 0 or 1 No prior systemic therapy for ES-SCLC Treatment-free for at least 6 months since last chemo/radiotherapy, among those treated (with curative intent) with prior chemo/radiotherapy for limited-stage SCLC Histologically or cytologically confirmed ES-SCLC Adequate hematologic and end-organ function to receive 4 cycles of induction treatment with carboplatin, etoposide and atezolizumab Measurable disease, as defined by RECIST v1.1 Negative HIV test and no evidence of active Hepatitis B or Hepatitis C at screening  Exclusion Criteria for the Induction Phase:  CNS metastases Active or history of autoimmune disease or deficiency History of malignancies other than SCLC within 5 years prior to enrollment Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including anti-CTLA-4, anti-PD-1, and anti-PD-L1 therapeutic antibodies, or lurbinectedin or trabectedin History of idiopathic pulmonary fibrosis, organizing pneumonia, drug induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan Treatment with investigational therapy within 28 days prior to enrollment  Inclusion Criteria for the Maintenance Phase:  ECOG PS of 0 or 1 Ongoing response or stable disease per RECIST 1.1 after 4 cycles of induction therapy Toxicities attributed to prior induction anti-cancer therapy or PCI resolved to Grade <=1 Adequate hematologic and end-organ function  Exclusion Criteria for the Maintenance Phase:  CNS metastases Receiving consolidative chest radiation Severe infection within 2 weeks prior to randomization into the maintenance Treatment with therapeutic oral or IV antibiotics at the time of randomization