Study to Assess the Safety, Pharmacokinetics, and Efficacy of Baloxavir Marboxil in Healthy Pediatric Participants From Birth to < 1 Year With Influenza-Like Symptoms

• Age from birth to < 1 year at screening
• Written informed consent for study participation obtained from participant's parents or legal guardian
• Parent/guardian willing and able to comply with study requirements, in the investigator's judgment
• Participants with a diagnosis of influenza virus infection confirmed by the presence of all of the following:
• In the investigator's judgement there is a clinical suspicion of influenza
• At least one respiratory symptom (either cough or coryza)

(b) Positive prescreening influenza test (RIDT or PCR) performed within 48 hours of screening

• Participants with a negative prescreening COVID-19 test (RAT or PCR) within 48 hours of screening
• The time interval between the onset of symptoms and screening is ≤ 96 hours (the onset of symptoms is defined as the time when body temperature first exceeded 37.5°C if known, or the time when the first symptom was noticed by the parent or caregiver)

• Hospitalized for complications of influenza or significant comorbidities
• Concurrent infections requiring systemic antiviral therapy at screening
• Require, in the opinion of the investigator, any of the prohibited medication during the study
• Preterm neonates (born at < 37 weeks gestation) and/or weighing < 2.5 kg at screening
• Previous treatment with peramivir, laninamivir, oseltamivir, zanamivir, or amantadine within 2 weeks prior to screening
• Immunization with a live/attenuated influenza vaccine during the 2 weeks prior to screening
• 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 participants with known chronic renal failure
• Active cancer at any site
• History of organ transplant
• Known hypersensitivity to study drug (i.e., baloxavir marboxil) or to acetaminophen
• Participation in a clinical trial within 4 weeks or five half-lives of exposure to an investigational drug prior to screening, whichever is longer