This study consists of two parts: Part A Surveillance and Part B Transmission.
The main purpose of Part A is to evaluate the prevalence of pre-dose and treatment-emergent amino acid substitutions in pediatric participants' \<12 years with influenza treated with baloxavir marboxil.
Part B will include a subset of Part A participants who have household contacts (HHCs) recruited to the study. Part B will evaluate the incidence of onward influenza transmission from pediatric index participants (IPs) under 5 years of age and those aged 5 to under 12 years, treated with baloxavir marboxil, to their HHCs. Participants will no longer be enrolled to Part B Transmission as per protocol version 3.
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
Eligible to take part in 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