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

Inclusion Criteria:

- 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:

- Fever ≥ 38°C (tympanic temperature) at screening

- At least one respiratory symptom (either cough or coryza)

- 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)

Exclusion Criteria:

- 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 allergy 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