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

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

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