A Study to Confirm Efficacy of a Single Dose of Baloxavir Marboxil in the Post-exposure Prophylaxis (PEP) of Influenza Virus Infection in Chinese Participants

Inclusion Criteria: Index Participants (Participants With Influenza Virus Infection): * First participant in a household with influenza virus infection in the current influenza season * Fewer than 6 HHCs * Agreement by all household members to participate in the study and comply with the study protocol for the duration of the study * Diagnosed as having influenza with positive rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR) result * Onset of symptoms within 48 hours or less at time of signing Informed Consent Form (ICF)/Assent Form * PCR (-) or antigen test (-) for severe acute respiratory virus-coronavirus 2 (SARS-CoV-2) using point-of-care or local laboratory test HHCs of Index Participants: * Lived with the index participant for 48 hours or more prior to informed consent/assent * Meet all the following criteria and are judged not to have influenza virus infection by the investigator: body temperature (tympanic) \<37.5 degrees Celsius (°C); no influenza-like symptoms (cough, sore throat, headache, nasal discharge/nasal congestion, feverishness or chills, muscle or joint pain, or fatigue) at screening; negative RIDT or PCR result for influenza * For participants 12 years of age or older, ability to evaluate influenza symptoms by using a participant influenza symptoms diary; For participants under 12 years of age, capability of parents/caregivers to evaluate influenza symptoms by using a participant influenza symptoms diary * PCR (-) or antigen test (-) for SARS-CoV-2 using point-of-care or local laboratory test Exclusion Criteria: HHCs of Index Participants: * Diagnosed with influenza during the current influenza season * Unable to live with the index patient from screening until Day 10 * Live with a household member who has any influenza-like symptom(s) other than the index participant on the day of screening * Live with household members other than the index participant who was diagnosed with or strongly suspected to have influenza during the current influenza season * Have any underlying diseases requiring systemic (oral or injection), or nasal treatment of antipyretics/analgesics, corticosteroids, or immunosuppressive agents * Immunocompromised * Use of any medications with anti-influenza effect within 30 days prior to screening * Known allergy and/or history of significant intolerance against baloxavir marboxil * Exposure to an investigational drug within 30 days or 5 half-lives of the drug prior to screening, whichever is longer Note: Other protocol-defined inclusion/exclusion criteria may apply