Study to Assess the Efficacy of Baloxavir Marboxil Versus Placebo to Reduce Onward Transmission of Influenza A or B in Households

INCLUSION CRITERIA: Index Patients (IPs): * Able to comply with the study protocol per investigator judgment. * Diagnosed with acute influenza infection by investigator. * Polymerase chain reaction \[PCR\] (+) or Rapid Influenza Diagnostic Test \[RIDT\] (+) for influenza A/B based on cobas® SARS-CoV-2 and influenza A/B or other point-of-care / local laboratory results. * PCR (-) or antigen test (-) for SARS-CoV-2 based on cobas® SARS-CoV-2 and Influenza A/B test or other point-of-care / local laboratory result * Presence of (a) fever (\>=38.0 °C per tympanic or rectal thermometer; \>=37.5 °C per axillary, oral or forehead/temporal thermometer) or (b) any influenza symptoms (cough, sore throat, nasal congestion, headache, feverishness or chills, muscle or joint pain, fatigue). * The time interval between the onset of fever or influenza symptoms and the pre-dose examinations is 48 hours or less. * IP lives in a household where: (1) No HHC is known to have been diagnosed with influenza or SARS-CoV-2 infection by a healthcare professional (HCP) in the past 4 weeks; (2) All HHCs are expected to meet the key HHC inclusion criteria; (3) \>=1 HHCs are expected to participate in the full study who have not received the influenza vaccine within 6 months prior to screening. * Women of childbearing potential: Agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures specified in the protocol All HHCs (Part 1): * PCR (-) or RIDT (-) based on c…