Study to Assess the Safety, Pharmacokinetics, and Efficacy of Baloxavir Marboxil in Chinese Pediatric Participants 1 to <12 Years of Age With Influenza Symptoms

Inclusion Criteria:

• A participant who has a diagnosis of influenza virus infection and meets all thefollowing conditions:

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

• At least one of the respiratory symptoms of influenza virus infection

• A rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR) showspositive for influenza A/B, e.g., point-of-care/local laboratory results with use ofnasal aspirate, throat swab, or nasal drip/droplet (or other appropriate sample)

• The time interval between the onset of symptoms and screening is ≤ 48 hours

• PCR (-) or antigen test (-) for severe acute respiratory virus-coronavirus 2 (SARS-CoV-2) using point-of-care/local laboratory test with nasal aspirate, throatswab, or nasal drip/droplet (or other appropriate sample)

Exclusion Criteria:

• A participant having severe influenza virus infection symptoms requiring inpatienttreatment

• Received systemic corticosteroid or immunosuppressive therapy

• Primary immunodeficiency syndrome

• History of organ transplantation

• Human immunodeficiency virus (HIV) infection

• Immunization with a live/attenuated influenza vaccine in 2 weeks prior torandomization

• Previous malignancy within the last 5 years or has an active cancer at any site

• A participant who received any medications with anti-flu effect such as baloxavir,peramivir, oseltamivir, zanamivir, favipiravir, arbidol, amantadine or traditionalChinese anti-influenza medicines within 30 days before screening

• Diagnosed with or suspected SARS-CoV-2 infection, or close contacts of diagnosed orsuspected SARS-CoV-2 infected patients

• Severe underlying disease or condition potentially affecting study evaluation in theopinion of the investigator/sub-investigator

• A participant who received an investigational or unapproved drug product within 30days or 5 x the half-life before screening, whichever is longer