An adverse event (AE) is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease temporally associated with the use of a medicinal (investigational) product, whether or not considered related to the medicinal (investigational) product. A serious adverse event (SAE) is any significant hazard, contraindication, side effect that is fatal or life-threatening, requires hospitalization or prolongation of an existing hospitalization, results in persistent or significant disability/ incapacity, is a congenital anomaly/ birth defect, is medically significant or requires intervention to prevent one or other of the outcomes listed above.

Results provided by body-weight groups for participants in the Baloxavir Marboxil arm. Values below lower limit of quantification (0.5 ng/mL) are set to zero.

The pharmacokinetic (PK) population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point. Sparse PK population comprises all participants in the PK population who did not provide informed consent to intensive PK sampling

Results provided by body-weight groups for participants in the Baloxavir Marboxil arm.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point. Sparse PK population comprises all participants in the PK population who did not provide informed consent to intensive PK sampling

Results provided by body-weight groups for participants in the Baloxavir Marboxil arm. Values below lower limit of quantification (0.5 ng/mL) are set to zero.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point. Extensive PK population comprises all participants in the PK population who provided informed consent to intensive PK sampling (additional time points for sample collection on Day 1)

Results provided by body-weight groups for participants in the Baloxavir Marboxil arm. Values below lower limit of quantification (0.5 ng/mL) are set to zero.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point. Extensive PK population comprises all participants in the PK population who provided informed consent to intensive PK sampling (additional time points for sample collection on Day 1)

Time to alleviation of influenza signs and symptoms is defined as the length of time taken from the start of treatment to the point at which all of the following criteria are met and remain so for at least 21.5 hours:

• A score of 0 (no problem) or 1 (minor problem) for cough and nasal symptoms (items 14 and 15 of the Canadian Acute Respiratory Illness and Flu Scale [CARIFS])
• A "yes" response to the following question on the CARIFS: "Since the last assessment has the subject been able to return to day care/school, or resume his or her normal daily activity in the same way as performed prior to developing the flu?"
• First return to afebrile state (tympanic temperature ≤37.2 degree Celsius [°C])

Includes all participants with CARIFS Assessment who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study

Length of time taken by participants to return to afebrile state [tympanic temperature ≤ 37.2°C] and remaining so for at least 21.5 hours.

Includes all participants with CARIFS Assessment who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study.

The clinical efficacy of baloxavir marboxil is evaluated by duration of symptoms i.e., alleviation of all symptoms as defined by a score of 0 [no problem] or 1 [minor problem] and remaining so for at least 21.5 hours, for all 18 symptoms specified in the CARIFS questionnaire.

Includes all participants with CARIFS Assessment who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study.

Time to Return to Normal health and activity' is identified by a 'Yes' response to the following question on the CARIFS: "Since the last assessment has the patient been able to return to day care/school, or resume his or her normal daily activity in the same way as performed prior to developing the flu?"

Includes all participants with CARIFS Assessment who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study.

Influenza related complications include death, hospitalization, radiologically confirmed pneumonia, bronchitis, sinusitis, otitis media, encephalitis/encephalopathy, febrile seizures, myositis.

Includes all participants who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study

Influenza related complications include death, hospitalization, radiologically confirmed pneumonia, bronchitis, sinusitis, otitis media, encephalitis/encephalopathy, febrile seizures, myositis.

Includes all participants who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study

Includes all participants who have had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study

Time to cessation of viral shedding by virus titer is defined as the time, in hours, between the initiation of any study treatment and first time when the influenza virus titer is below the limit of detection.

Includes all participants with post-baseline Virology assessment and a positive virus titer on Day 1

Time to cessation of viral shedding by RT-PCR, in hours, is defined as the time between the initiation of any study treatment and first time when the virus RNA by RT-PCR is below the limit of detection.

Includes all participants with post-baseline Virology assessment and a positive virus RNA by RT-PCR on Day 1. Participants whose virus RNA did not reach the limit by the last observation time point are treated as censored at that time point.

Influenza virus titer (log10TCID50/ML) is the quantity of influenza virus in a given volume within the samples obtained from nasal swabs. If influenza virus titer was less than the lower limit of quantification, the virus titer was imputed as 0.749 (log10TCID50/mL). A lower value indicates lower viral titer.

Includes all participants with a positive virus titer on Day 1

If the amount of virus RNA was less than the lower limit of quantification, the amount of virus RNA was imputed as 2.18 for flu A and 2.93 for flu B (log10 virus particles/mL)

Includes all participants with positive virus RNA by RT-PCR on Day 1

Includes all participants with a positive virus titer on Day 1

Includes all participants with positive virus RNA by RT-PCR on Day 1

Area under the curve (AUC) in virus titer was calculated using the trapezoidal method.

Includes all participants with post-baseline Virology assessment and a positive virus titer on Day 1

AUC in virus RNA (RT-PCR) is defined as AUC of change from baseline in the amount of virus RNA (RT-PCR) from Day 1 to Day 10. AUC is calculated using the trapezoidal method similar to AUC in virus titer.

Includes all participants with positive virus RNA by RT-PCR on Day 1 and at least 1 post-baseline test.

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point

Dose groups correspond to body-weight groups. 2mg/kg dose was used for subjects <20 kgs and 40 mg dose was used for subjects >20 kgs.

The PK population consists of all participants that have at least one post-dose drug concentration measurement at a scheduled visit time point