Study to Assess the Safety, Pharmacokinetics, and Efficacy of Baloxavir Marboxil in Healthy Pediatric Participants With Influenza-Like Symptoms

Inclusion Criteria: * Aged 1 to \< 12 years at randomization (Day 1). * Written informed consent/assent for study participation obtained from participant's parents or legal guardian, with assent as appropriate by the participant, depending on the patient's level of understanding * Participant able to comply with study requirements, depending on the patient's level of understanding * Participant with a diagnosis of influenza virus infection confirmed by the presence of all of the following: * Fever ≥ 38 degree celsius (tympanic temperature) at screening * At least one respiratory symptom (either cough or nasal congestion) * The time interval between the onset of symptoms and screening is ≤ 48 hours Exclusion Criteria: * Severe symptoms of influenza virus infection requiring inpatient treatment * Concurrent infections requiring systemic antiviral therapy at screening * Require, in the opinion of the investigator, any of the prohibited medication during the study * Previous treatment with peramivir, laninamivir, oseltamivir, zanamivir, or amantadine within 2 weeks prior to screening * Immunization with a live/attenuated influenza vaccine in the 2 weeks prior to randomization * Concomitant treatment with steroids or other immuno-suppressant therapy * Known HIV infection or other immunosuppressive disorder * Uncontrolled renal, vascular, neurologic, or metabolic disease (e.g., diabetes, thyroid disorders, adrenal disease), hepatitis, cirrhosis, or pulmonary disease or participa…