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 parentsor legal guardian

• Parent/guardian willing and able to comply with study requirements, in theinvestigator's judgment

• Participants with a diagnosis of influenza virus infection confirmed by the presenceof all of the following:

1. In the investigator's judgement there is a clinical suspicion of influenza

2. At least one respiratory symptom (either cough or coryza)

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

• Participants with a negative prescreening COVID-19 test (RAT or PCR) within 48 hoursof screening

• The time interval between the onset of symptoms and screening is ≤ 96 hours (theonset of symptoms is defined as the time when body temperature first exceeded 37.5°Cif 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 duringthe study

• Preterm neonates (born at < 37 weeks gestation) and/or weighing < 2.5 kg atscreening

• Previous treatment with peramivir, laninamivir, oseltamivir, zanamivir, oramantadine within 2 weeks prior to screening

• Immunization with a live/attenuated influenza vaccine during the 2 weeks prior toscreening

• 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 orparticipants 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 toan investigational drug prior to screening, whichever is longer