Pembrolizumab/Vibostolimab (MK-7684A) or Atezolizumab in Combination With Chemotherapy in First Line Treatment of Extensive-Stage Small Cell Lung Cancer (MK-7684A-008/KEYVIBE-008)

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Has histologically or cytologically confirmed diagnosis of extensive-stage small cell lung cancer (ES-SCLC) in need of first-line therapy * Has ES-SCLC defined as Stage IV (T any, N any, M1a/b/c) by the American Joint Committee on Cancer, Eighth Edition or T3-T4 due to multiple lung nodules that are too extensive or have tumor/nodal volume that is too large to be encompassed in a tolerable radiation plan * Males agree to use contraception, refrain from donating sperm, and abstain from heterosexual intercourse * Females are not pregnant or breastfeeding, is not a woman of childbearing potential (WOCBP) or is a WOCBP who uses a highly effective contraceptive method, or is abstinent from heterosexual intercourse * Has measurable disease per Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 * Has a predicted life expectancy of \>3 months Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Is considered a poor medical risk due to a serious, uncontrolled medical disorder or non-malignant systemic disease * Has received prior treatment for Small Cell Lung Cancer (SCLC) * Is expected to require any other form of antineoplastic therapy for SCLC while on study * Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention * Has received an investigational agent or has used an investigational device within 4 weeks prior to study intervention administration * Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior the first dose of study medication * Has a known additional malignancy that is progressing or has required active treatment within the past 3 years * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis * Has a history of severe hypersensitivity reaction (≥Grade 3) to any study intervention and/or any of its excipients * Has an active autoimmune disease that has required systemic treatment in past 2 years * Has a history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease * Has a known history of, or active, neurologic paraneoplastic syndrome * Has an active infection requiring systemic therapy * Has a known history of human immunodeficiency virus (HIV) infection * Has a known history of Hepatitis B or known active Hepatitis C virus infection * Has had an allogenic tissue/solid organ transplant * Has had major surgery within prior 3 weeks or has not recovered adequately from toxicity and/or complications from an intervention prior to receiving the first dose of study intervention * Has symptomatic ascites or pleural effusion