Study to Assess the Safety, Pharmacokinetics, and Efficacy of Baloxavir Marboxil in Chinese Pediatric Participants 1 to <12 Years of Age With Influenza Symptoms

Inclusion Criteria: * A participant who has a diagnosis of influenza virus infection and meets all the following conditions: * Fever ≥ 38°C (tympanic temperature) at screening * At least one of the respiratory symptoms of influenza virus infection * A rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR) shows positive for influenza A/B, e.g., point-of-care/local laboratory results with use of nasal aspirate, throat swab, or nasal drip/droplet (or other appropriate sample) * The time interval between the onset of symptoms and screening is ≤ 48 hours * PCR (-) or antigen test (-) for severe acute respiratory virus-coronavirus 2 (SARS-CoV-2) using point-of-care/local laboratory test with nasal aspirate, throat swab, or nasal drip/droplet (or other appropriate sample) Exclusion Criteria: * A participant having severe influenza virus infection symptoms requiring inpatient treatment * Received systemic corticosteroid or immunosuppressive therapy * Primary immunodeficiency syndrome * History of organ transplantation * Human immunodeficiency virus (HIV) infection * Immunization with a live/attenuated influenza vaccine in 2 weeks prior to randomization * Previous malignancy within the last 5 years or has an active cancer at any site * A participant who received any medications with anti-flu effect such as baloxavir, peramivir, oseltamivir, zanamivir, favipiravir, arbidol, amantadine or traditional Chinese anti-influenza medicines within 30 days before scre…