Clinical response is defined as the resolution of at least 4 of the 5 vital signs (temperature, oxygen saturation, respiratory status, heart rate, systolic blood pressure) within the respective resolution criteria, maintained for at least 24 hours, or hospital discharge, whichever occurred first. This analysis was performed for Influenza positive population, for those with symptom onset less than or equal to (<=) 4 days, and for those on mechanical (mech) ventilation or in intensive care unit (ICU). 99 days is censored time for the participants who did not achieve TTCR.

ITT-E population comprised of all randomized participants who received at least one dose of investigational product. The Influenza positive population (IPP) is comprised of all participants in the ITT-E population with proven influenza infection.

Respiratory Status (RS) is a component of TTCR. Response criteria included the return to the pre-morbid oxygen requirement (participants with chronic oxygen use), a need for supplemental oxygen (administered by any modality: ventilator, non-invasive ventilation, facemask, facetent, nasal canula, etc.) to no need for supplemental oxygen, or a respiratory rate of =<24 breaths/minute (without supplemental oxygen). Data are presented as the percentage of participants achieving respiratory improvement.

IPP Population

The number of participants who died on or before Day 14, Day 28, and the End of Study Visit were summarized.

IPP Population

The Katz ADL scores were collected for bathing, dressing, toileting, transferring, continence, and feeding activities and were assessed once daily during the treatment period/hospitalization and once at each post-treatment Clinic Visit. For the six individual activities, a score of 1 indicates independence, and a score of 0 indicates dependence. The total score is generated by adding the scores of all six activities. A total score of 6 indicates that the participant was independent; a total score of 0 indicates that the participant was very dependent. Baseline is defined as the pre-dose value collected on Study Day 1. Change from Baseline is defined as the difference at each time point (Day 5/6, and Day 10/11, and last day S/R if treatment was extended beyond 5 days) and the end of the study (post-treatment [PT] +28 Days) compared to Baseline.

IPP population. Only those participants available at the specified time points were analyzed.

Pre-morbid functional status is defined as the best functional status in the 4 weeks prior to enrolment. Median time to return to pre-morbid functional status was assessed via the Katz ADL score (bathing, dressing, toileting, transferring, continence, and feeding activities). For the six individual activities, a score of 1 indicates independence, and a score of 0 indicates dependence. The total score is generated by adding the scores of all six activities. A total score of 6 indicates that the participant was independent; a total score of 0 indicates that the participant was very dependent.

IPP population. Only those participants available at the specified time points were analyzed.

Pre-morbid functional status is defined as the best functional status in the 4 weeks prior to enrolment. The number of participants who returned to their pre-morbid functional status at the end of the study assessed per the Katz ADL score (bathing, dressing, toileting, transferring, continence and feeding activities) is summarized.

IPP Population

Median time to return to pre-morbid level of activity was assessed once daily during treatment/hospitalization and once at each post-treatment assessment and was measured using the 3- point scale (bed rest, limited ambulation, or unrestricted).

IPP Population. Participants succeeded in pre-morbid functional status were analyzed.

Influenza clinical symptoms included nasal symptoms (rhinorrhea, congestion), feverishness, cough, myalgias, fatigue, diarrhea, anorexia, dyspnea, headache, sore throat, nausea, and vomiting. Influenza symptoms were assessed once daily during inpatient hospitalization and once at each post-treatment assessment.

IPP Population

Influenza clinical symptoms included nasal symptoms (rhinorrhea, congestion), feverishness, cough, myalgias, fatigue, diarrhea, anorexia, dyspnea, headache, sore throat, nausea, and vomiting. Influenza symptoms were assessed once daily during inpatient/hospitalization and once at each post-treatment assessment.

IPP Population. Only those participants available at the specified time points were analyzed.

The number of participants with complications of influenza and associated antibiotic use were summarized

IPP Population

Ventilation status was assessed three times daily during the treatment period/hospitalization. Ventilation status was assessed once daily during inpatient/hospitization and once at each post-treatment clinic visit. The number of participants reported for machine-assisted: extracorporeal membrane oxygenation (ECMO), endotracheal mechanical ventilation, and supplemental oxygen delivery (SOD), no supplemental oxygen (O2) or ventilation support, Respiratory support at "any time (AT) on study" and at Baseline (Day 1) are summarized. Data for the "any time (AT) on study" time point was reported.

IPP Population

Ventilation status was assessed three times daily during the treatment period/hospitalization. Ventilation status was assessed once daily during inpatient/hospitalization and once at each post-treatment clinic visit.

IPP Population. Only those participants available with the indicated ventilator support or oxygen supplementation were analyzed.

Hospital duration and ICU duration was assessed from the first day of dosing. Hospital duration was calculated as the discharge date minus the admission date + 1. Hospital duration while on study was the earlier of discharge, completion, or withdrawal minus the later of the admission date or the study start date + 1. ICU duration-Modified was calculated as the original ICU duration minus ICU days prior to Study Day 1.

IPP population. Only those participants available at the specified time points were analyzed.

The absence of fever is defined as a non-axillary temperature recording <=36.6 degrees Celsius axillary, <= 37.2 degrees Celsius oral or <= 37.7 degrees Celsius core. Respiratory Status (RS) response criteria included the return to the pre-morbid oxygen requirement (participants with chronic oxygen use), or the need for supplemental oxygen (administered by any modality: ventilator, non-invasive ventilation, facemask, facetent, nasal canula, etc.) to no need for supplemental oxygen, or a respiratory rate =<24 breaths/minute (without supplemental oxygen). Oxygen saturation response criteria: >=95% (without supplemental oxygen). Heart rate response criteria: =<100 beats/minute. Systolic blood pressure response criteria: >=90 millimeters of mercury. Vital signs were assessed three times daily during the treatment period/hospitalization. Vital signs were assessed once daily during inpatient/hospitization and once at each post-treatment clinic visit.

IPP Population

Virologic improvement is defined as a 2 log drop in viral load or sustained undetectable viral ribonucleic acid (RNA) (on two successive occasions) as measured by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) from nasopharyngeal samples. Nasopharyngeal swabs were collected daily from Baseline through Day 5. If randomized treatment was continued beyond Day 5, samples were taken on Treatment Days 6, 8, 10 and on the last day of randomized treatment. For participants who utilized the Switch (S)/Rescue (R) option, samples were taken on S/R Day 1, S/R Day 3, S/R Day 5, or S/R Day 6, whichever was the last day of S/R treatment. Nasopharyngeal swabs were taken if the participant was symptomatic and continued to be hospitalized on the Post-Treatment +2, +5, +9, +16, and +28 day assessment.

IPP Population. Only those participants available at the specified time points were analyzed. Data presented is for participants positive at Baseline.

Nasopharyngeal swabs were collected daily from Baseline through Day 5. If randomized treatment was continued beyond Day 5, samples were taken on Treatment Day 6, Day 8, Day 10, Day 11, and the last day of randomized treatment. For participants who utilized the S/R option, samples were taken on S/R Day1, S/R Day3, S/R Day5, or S/R Day6, whichever was the last day of S/R treatment. Samples were taken if the participant was symptomatic and continued to be hospitalized on the Post-Treatment +2, +5, +9, +16 and +28Day assessment. Viral load was measured by Quantitative Virus Culture, log10 50% Tissue Culture Infectious Dose (TCID50)/milliliter (mL). Baseline is defined as the pre-dose value collected on Study Day 1. Change from Baseline was calculated as the post-Baseline value minus Baseline value .

IPP Population. Only those participants available at the specified time points were analyzed.

Nasopharyngeal swabs were collected daily from Baseline through Day 5. If randomized treatment was continued beyond Day 5, samples were taken on Treatment Day 6, Day 8, Day 10, Day 11, and the last day of randomized treatment. For participants who utilized the S/R option, samples were taken on S/R Day 1, S/R Day 3, S/R Day 5, or S/R Day 6, whichever was the last day of S/R treatment. Samples were taken if the participant was symptomatic and continued to be hospitalized on the post-treatment +2, +5, +9, +16 and +28 day assessment. Viral load as measured by PCR. Baseline is defined as the pre-dose value collected on Study Day 1. Change from Baseline was calculated as the post-Baseline value minus the Baseline value.

IPP Population. Only those participants available at the specified time points were analyzed (represented by n=X, X, X in the category titles)

Lower respiratory samples included BAL and endotracheal aspirates. Endotracheal aspirates were requested in participants (par.) who were intubated. Upper (nasopharyngeal swabs) and lower (Endotracheal aspirates, bronchoalveolar lavage samples) respiratory samples were collected daily from Baseline/Day 1 through Day 5 and Day 6 (if the last day of randomized treatment [trt]). Endotracheal aspirates were collected in participants who were intubated. If trt was continued beyond Day 5, additional samples were taken on Trt Day 6, Day 8, Day 10, and/or the day of the last dose of randomized trt, if applicable, and S/R Day 1, S/R Day 3, S/R Day 5, or S/R Day 6 if the last day of S/R trt. If the par. was symptomatic and hospitalized, samples were taken on the Post-Trt +2, +5, +9, +16 assessment days, and at the Post-Trt [PT]+28 Day assessment. Assessment of samples was done by quantitative RT-PCR and viral culture.

IPP Population. Data is presented for participants positive at Baseline. Only those participants available at the specified time points were analyzed.

Upper (nasopharyngeal swabs) and lower (Endotracheal aspirates, bronchoalveolar lavage samples, where available) respiratory samples were collected daily from Baseline/Day 1 through Day 5 and Day 6 (if the last day of randomized treatment). Endotracheal aspirates were collected in participants who were intubated. If treatment was continued beyond Day 5, additional samples were taken on Treatment Day 6, Day 8, Day 10, and/or the day of the last dose of randomized treatment, if applicable, and S/R Day 1, S/R Day 3, S/R Day 5, or S/R Day 6 if the last day of S/R treatment. If the participant was symptomatic and hospitalized, samples were taken on the Post-treatment+2, +5, +9, +16 assessment days, and at the Post-Treatment +28 Day. Assessment of samples was done by quantitative RT-PCR.

IPP Population. Only those participants available at the specified time points were analyzed. Data also presented for participants positive at Baseline.

Nasopharyngeal swabs and endotracheal /BAL samples were collected for viral susceptibility analysis. Susceptibility analyses consisted of phenotyping and genotyping. Resistance mutations were detected by genotyping. Viral susceptibility to zanamivir and oral oseltamivir at Baseline and throughout treatment determined by NA and HA (gene of influenza A and B viruses) sequence analysis and NA enzyme inhibition. Number of participants with viral mutation events are summarized, this includes all resistance mutations (substitutions) i.e. those present at Baseline and those that emerged during treatment.

IPP Population

An AE is defined as any untoward medical occurrence in a participant temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a medicinal product. For marketed medicinal products, this also includes failure to produce expected benefits (i.e., lack of efficacy), abuse, or misuse. All AEs were assessed by the Investigator as related or not related to the study treatment.

The Safety Population comprised of all randomized participants who received at least one dose of investigational product and assessed according to their actual treatment received, regardless of the randomization assigned.

An AE is defined as any untoward medical occurrence in a participant temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a medicinal product. For marketed medicinal products, this also includes failure to produce expected benefits (i.e., lack of efficacy), abuse, or misuse. AEs that occurred during the study were evaluated by the Investigator and graded according to the Division of Acquired Immunodeficiency Syndrome (DAIDS) table for grading the severity of AEs. Grade 3=severe; Grade 4=potentially life threatening.

Safety population

An AE is defined as any untoward medical occurrence in a participant temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a medicinal product. For marketed medicinal products, this also includes failure to produce expected benefits (i.e., lack of efficacy), abuse, or misuse.

Safety Population

An AE is defined as any untoward medical occurrence in a participant temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a medicinal product. For marketed medicinal products, this also includes failure to produce expected benefits (i.e., lack of efficacy), abuse, or misuse.

Safety Population

An AE is defined as any untoward medical occurrence in a participant temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a medicinal product. For marketed medicinal products, this also includes failure to produce expected benefits (i.e., lack of efficacy), abuse, or misuse. AEs that occurred during the study were evaluated by the Investigator and graded according to the DAIDS table for grading the severity of adult and pediatric AEs. Grade 3=severe; Grade 4=potentially life threatening. All AEs were assessed by the Investigator as related or not related to the study treatment.

Safety Population

Samples for laboratory assessments were collected at Baseline (Day 1), Day 3, Day 5/6, Day 8, Day 10/11 (or last day of randomized treatment), switch/rescue (S/R) Day 1, S/R Day 3, and S/R Day 5/6 (last day of S/R treatment for those participants who utilized this option), Post-Treatment +2 (if hospitalized), and Post-Treatment +5, +16, and +28 Days. Clinical chemistry parameters included albumin, alkaline phosphatase (ALP), alanine amino transferase (ALT), aspartate amino tranferase (AST), total bilirubin, calcium, creatine kinase, chloride, carbon dioxide content (CO2), creatinine, potassium, magnesium, sodium. Per the DAIDS table for grading the severity of adult and pediatric AEs, Grade (G) 1=mild, G2= moderate, G3=severe and G4=potentially life threatening. The number of participants with values that were G1, G2, G3 and G4 relative to the normal range are summarized.

Safety Population. Only the participants available at the time of assessment were analyzed.

Blood samples for laboratory assessments were collected at Baseline (Day 1), Day 3, Day 5/6, Day 8, Day 10/11 (or last day of randomized treatment), S/R Day 1, S/R Day 3, and S/R Day 5/6 (last day of S/R treatment for those participants who utilized this option), Post-Treatment +2 (if hospitalized), and Post-Treatment +5, +16, and +28 Days. Hematology parameters included hemoglobin, lymphocytes, total neutrophils, platelet count, and white blood cell (WBC) count. Per the DAIDS table for grading the severity of adult and pediatric AEs, Grade (G) 1=mild, G2= moderate, G3=severe and G4=potentially life threatening. The number of participants with values that were G1, G2, G3 and G4 relative to the normal range for the indicated hematology parameters is summarized. Baseline is defined as the pre-dose value collected on Study Day 1.

Safety Population. Only the participants available at the time of assessment were analyzed.

A toxicity was considered to be TE if it was greater than the Baseline grade, and if it had developed or increased post-Baseline in intensity (and prior to the last dose of investigational product). Clinical chemistry parameters included albumin, ALP, ALT, AST, total bilirubin, calcium, creatine kinase, chloride, CO2/bicarbonate, creatinine, potassium, magnesium and sodium. Per the DAIDS table for grading the severity of adult and pediatric AEs, Grade 3=severe and Grade 4=potentially life threatening. Baseline is defined as the pre-dose value collected on Study Day 1.

Safety population. Only the participants available at the time of assessment were analyzed.

A toxicity was considered to be TE if it was greater than the Baseline grade, and if it had developed or increased post-Baseline in intensity (and prior to the last dose of investigational product). The hematology parameters included hemoglobin, lymphocytes, total neutrophils, platelet count, and WBC count. Per the DAIDS table for grading the severity of adult and pediatric AEs, Grade 3=severe and Grade 4=potentially life threatening. Baseline is defined as the pre-dose value collected on Study Day 1.

Safety Population. Only the participants available at the time of assessment were analyzed.

Oxygen delivery were assessed three times daily at Baseline (Day 1) and during the treatment period/hospitalization (ideally at least 6 hours apart) and once daily during inpatient/hospitalization and once at Post +5 days, +16 days, and +28 days clinic visits. The median quantity of oxygen delivery during the study was not summarized since the data was not collected in a way to accurately calculate values. Baseline is defined as the pre-dose value collected on Study Day 1.

This end point was not analyzed

On Baseline/Day 1, a 12-lead ECG was obtained within approximately 24 hours prior to dosing. The number of participants with an ECG status of normal and abnormal CS or NCS, as determined by the Investigator, is reported. Normal=all ECG parameters within the accepted normal ranges. Abnormal=ECG findings outside of normal ranges. CS=ECG with a CS abnormality that meets exclusion criteria. NCS=ECG with an abnormality that is not CS nor meets exclusion criteria, per Investigator, based on reasonable standards of clinical judgment. In the original protocol ECGs were also done on Day 4, however, amendment 2 removed this requirement and therefore not all participants had Day 4 ECGs.

Safety Population. Only those participants available at the specified time points were analyzed.

Pharmacokinetic samples were collected at four time points to characterize peak concentration (end of infusion; C[EOI]) after the first dose on Day 1 and on Day 4 to characterize the pre-dose concentration (C[0]), the peak concentration C(EOI), and the trough concentration at 11-12 hours post-dose (C[12]) of zanamavir. Data was summarized by Creatinine clearance (CL) Category. The dose on Day 1 is the initial dose (unadjusted) and the dose on Day 4 is the maintenance dose.

Pharmacokinetic (PK) Population comprised of all participants who received IV zanamivir and underwent sparse PK sampling during the study from which one or more serum zanamivir concentrations was determined. This outcome was not analyzed for participants receiving oseltamivir 75 mg. Only the participants available at the time point were analyzed.