An 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 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.

Safety-evaluable population included all participants who received at least one dose of treatment regardless of whether they had any follow-up assessments.

S-033447 is an active metabolite of baloxavir marboxil.

Pharmacokinetics (PK)-evaluable population included all participants in the ITT population who had at least one post-dose drug concentration measurement at a scheduled visit timepoint. Overall number analyzed is the number of participants with data available for analysis. Number analyzed per timepoint are unique number of participants out of all the assessed participants with data available for analysis at specified timepoint. Different participants may have contributed data for each timepoint.

S-033447 is an active metabolite of baloxavir marboxil.

PK-evaluable population included all participants in the ITT population who had at least one post-dose drug concentration measurement at a scheduled visit timepoint. Overall number analyzed is the number of participants with data available for analysis. Since baloxavir marboxil was barely measurable in plasma, baloxavir marboxil pharmacokinetic parameters were not determined.

S-033447 is an active metabolite of baloxavir marboxil.

PK-evaluable population included all participants in the ITT population who had at least one post-dose drug concentration measurement at a scheduled visit timepoint. Overall number analyzed is the number of participants with data available for analysis. Since baloxavir marboxil was barely measurable in plasma, baloxavir marboxil pharmacokinetic parameters were not determined.

S-033447 is an active metabolite of baloxavir marboxil.

PK-evaluable population included all participants in the ITT population who had at least one post-dose drug concentration measurement at a scheduled visit timepoint. Overall number analyzed is the number of participants with data available for analysis. Since baloxavir marboxil was barely measurable in plasma, baloxavir marboxil pharmacokinetic parameters were not determined.

S-033447 is an active metabolite of baloxavir marboxil.

PK-evaluable population included all participants in the ITT population who had at least one post-dose drug concentration measurement at a scheduled visit timepoint. Overall number analyzed is the number of participants with data available for analysis. Since baloxavir marboxil was barely measurable in plasma, baloxavir marboxil pharmacokinetic parameters were not determined.

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

• A score of 0 (no problem) or 1 (minor problem) for cough and nasal symptoms for 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 participant 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]).

Median time was estimated from the Kaplan-Meier curve. Participants who withdrew prior to an event of interest or did not experience resolution of symptoms were censored at the last observation time point.

Intent-to-Treat Influenza-Infected (ITTi) population is a subset of ITT participants who had a laboratory confirmation of influenza infection (polymerase chain reaction [PCR] result) from any swab sample collected at baseline or during the study.

Duration of fever was defined as the 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 after onset. Participants who did not have fever at baseline or whose body temperature was not collected were excluded from the analysis. Median time was estimated from the Kaplan-Meier curve.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants available for analysis.

The efficacy of baloxavir marboxil was 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 (Poor appetite; Not sleeping well; Irritable, cranky, fussy; Feels unwell; Low energy, tired; Not playing well; Crying more than usual; Needing extra care; Clinginess; Headache; Sore throat; Muscle aches or pains; Fever; Cough; Nasal congestion, runny nose; Vomiting; Not interested in what's going on; Unable to get out of bed) specified in the CARIFS questionnaire. Median time was estimated from the Kaplan-Meier curve. Participants who withdrew prior to an event of interest or did not experience resolution of symptoms were censored at the last observation time point.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study.

Time to return to normal health and activity was defined 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?" and remaining so for at least 21.5 hours.

Median time was estimated from the Kaplan-Meier curve.Participants who withdrew prior to an event of interest or did not experience resolution of symptoms were censored at the last observation time point.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with data available for analysis

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

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study.

The number of participants who required antibiotics for influenza related complication are reported here.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study.

Time to cessation of viral shedding by virus titer was defined as the time, in hours, between the initiation of any study treatment and first time when the influenza virus titer was below the limit of detection (0.75 log10 tissue culture infectious dose (TCID)50/milliliters [mL]). Participants whose virus titers did not reach the limit by the last observation time point were treated as censored at that time point. One day was converted into 24 hours. Median time was estimated from the Kaplan-Meier curve.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with data available for analysis.

Time to cessation of viral shedding by RT-PCR, in hours, was defined as the time between the initiation of study treatment and first time when the virus ribonucleic acid (RNA) by RT-PCR qualitative result was negative (no cycle threshold [Ct]-value detectable). Participants who did not have a negative result by the last observation time point were treated as censored at that time point. For the participants with multiple virus types, this endpoint was defined as the time between the initiation of the study treatment and first time when the virus RNA by RT-PCR qualitative result was negative for all virus types. One day was converted into 24 hours. Median time was estimated from the Kaplan -Meier curve. Participants with a positive virus RNA on Day 1 are included in this analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with a positive virus RNA on Day 1.

Change from baseline in influenza virus titer (log10TCID50/mL) was defined as the change from baseline in influenza virus titer on Days 2, 4, 6, 10, and 29. If influenza virus titer was less than the lower limit of quantification (LLOQ), the virus titer was imputed as 0.749 (log10TCID50/mL). Only participants with a positive virus titer on Day 1 were included in this analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Number analyzed per timepoint are unique number of participants out of all the assessed participants with data available for analysis at specified timepoint. Different participants may have contributed data for each timepoint.

Change from baseline in the amount of virus RNA was defined as the change from baseline in the amount of virus RNA on Days 2, 4, 6 and 10. If the amount of virus RNA is less than the lower limit of quantification, the amount of virus RNA was imputed as the relevant LLOQ (log10 viral particles per mililiter (vp/mL)). If a participant was infected with multiple virus types, the sum of virus RNA (log10 vp/mL) was used for analysis. Participants with a positive virus RNA by RT-PCR on Day 1 were included in this analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Number analyzed per timepoint are unique number of participants out of all the assessed participants with data available for analysis at specified timepoint. Different participants may have contributed data for each timepoint.

Percentage of participants positive for influenza virus titer at each visit were defined as the percentage of participants whose influenza virus titer was not less than the LLOQ (0.75 log10TCID50/mL) or positive among those assessed for influenza virus titer on Days 2, 4, 6, 10 and 29. Participants with a positive influenza virus titer on Day 1 were included in this analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with a positive influenza virus titer on Day 1. Number analyzed is the number of participants with data available for analysis at the specified timepoint.

Percentage of participants positive by RT-PCR at each visit was defined as the percentage of participants with a positive qualitative result among those assessed by RT-PCR on Days 2, 4, 6, 10 and 29. Participants with a positive RT-PCR result on Day 1 were included in this analysis. Percentages are rounded off.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with a positive RT-PCR result on Day 1. Number analyzed is the number of participants with data available for analysis at the specified timepoint.

AUC in virus titer was calculated using the trapezoidal method. Twenty-four hours of time was converted into one day. Participants with a positive virus titer on Day 1 were included in this analysis. The LLOQ and lower limit of detection was defined as 0.75 log10TCID50/mL for flu A and 0.75 log10TCID50/mL for flu B. If a participant was infected with multiple virus types, the sum of those virus titers will be used for analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with a positive virus titer on Day 1.

AUC in virus RNA (RT-PCR) was defined as AUC of change from baseline in the amount of virus RNA (RT-PCR). AUC was calculated using the trapezoidal method similar to AUC in virus titer. Participants with a positive RT-PCR result on Day 1 were subjected to this analysis. If the amount of virus RNA is less than the lower limit of quantification, the amount of virus RNA was imputed as the relevant LLOQ (log10 viral particles per mililiter (vp/mL)). If a participant was infected with multiple virus types, the sum of those the amount of virus RNA was used for analysis.

ITTi population is a subset of ITT participants who had a laboratory confirmation of influenza infection (PCR result) from any swab sample collected at baseline or during the study. Overall number analyzed is the number of participants with a positive virus titer on Day 1.