Trials Today

(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

(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