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
participants with known chronic renal failure.

- Active cancer at any site

- History of organ transplantation

- Known allergy to either study drug (i.e., baloxavir marboxil and oseltamivir) or to
acetaminophen

- Females with child-bearing potential

- Participation in a clinical trial within 4 weeks or five half-lives of exposure to an
investigational drug prior to screening, whichever is longer