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 start of 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 HHC

\[B\] 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 HHC" criteria

\[D\] Full-study HHC:

* Fulfills the "partial HHC" 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 fulfil 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