This study looks at how well the medicine baloxavir marboxil works on kids aged 1 to less than 12 years old who have the flu. Baloxavir marboxil is a drug used to treat influenza, which is another name for the flu. Researchers want to see how the flu virus in kids reacts before and after taking a single dose of this medicine. To join, kids must have flu symptoms confirmed by a test within 24 hours and a negative COVID-19 test within 48 hours. Kids cannot have severe flu needing hospital care, be very sick from other conditions, or have taken certain flu medicines or experimental treatments recently.

• The study requires kids to have a flu diagnosis confirmed by a test.
• Participants cannot be severely sick or have taken certain medicines recently.
• This study involves checking the flu virus response to a single dose of medicine.