A Surveillance Study of Susceptibility to Baloxavir Marboxil in Pediatric Participants With Influenza and Transmission of Influenza to Household Contacts

Inclusion Criteria (Part A): * Participants with symptoms suggestive of influenza based on investigator's judgement with diagnosis confirmed by a positive local influenza test within 24 hours before full study screening * Participants with a negative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) test within 48 hours before full study screening * Time interval between onset of influenza symptoms and the pre-dose examinations at screening is 48 hours or less Inclusion Criteria (Part B): \[A\] IP: * Eligible to take part in Part A * Lives in a household with a HHC willing to be recruited as full household contact \[B\] HHCs: \- Each HHC living in the home of an IP at the time of IP treatment must be screened for partial or full-study HHC eligibility \[C\] Partial HHC: * Starts screening within 1 calendar day after IP treatment * Negative influenza and SARS-CoV-2 test at screening after IP's treatment with baloxavir marboxil * HHC lives in a household where 1 or more HHCs meet the full-study HHC requirements AND at least 1 full-study HHC has not received the influenza vaccine within 6 months prior to screening * HHC lives in a household where 1 or more HHCs meet the full-study HHC requirements AND at least 1 full-study HHC has not received the influenza vaccine within 6 months prior to screening. * HHC lives with other HHCs (if applicable) who fulfill all the "Partial household contact" criteria \[D\] Full-study HHC: * Fulfills the "Partial household contact" criteria * Agrees to participate in the full study * Will reside in the IP's house for at least 12 of the next 15 days and will be present for scheduled study visits * No influenza symptoms within 7 days prior to screening * Does not have a moderate or worse active infections OR infections requiring systemic or otherwise internally administered or otherwise internally administered antibiotic/antiviral/antifungal therapy Exclusion Criteria (Part A): * Participants with severe influenza virus infection requiring inpatient treatment * Severely immunocompromised participants \[including participants receiving immunosuppressant therapy, or those with cancer or human immunodeficiency virus (HIV) infection\] as defined by the investigator * Participants with concurrent (non-influenza) infections requiring systemic anti-microbial and/or anti-viral therapy at the pre-dose examinations * Treatment with baloxavir marboxil, peramivir, laninamivir, oseltamivir, zanamivir, rimantadine, umifenovir or amantadine within 30 days prior to screening * Treatment with an investigational influenza-specific monoclonal antibody within 6 months or 5 half-lives, whichever is longer, prior to screening * Treatment with an investigational therapy within 30 days or 5 half-lives, whichever is longer, prior to screening * Known hypersensitivity to baloxavir marboxil or the drug product excipients * Females who have commenced menarche (i.e., child-bearing potential) Exclusion Criteria (Part B): * IPs who fulfill an exclusion criterion for Part A * HHCs deemed to require influenza post-exposure prophylaxis with influenza antiviral treatment due to their risk of developing influenza-related complications in accordance with local guidelines or clinical practice * HHCs diagnosed with influenza by health care professional in the past 4 weeks