An adverse event is any untoward medical occurrence in a participant administered a pharmaceutical product and which does not necessarily have to have a causal relationship with the treatment. An adverse event can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding, for example), symptom, or disease temporally associated with the use of a pharmaceutical product, whether or not considered related to the pharmaceutical product. Preexisting conditions which worsen during a study are also considered as adverse events.

Safety Population included all randomized participants who received study drug, with participants grouped according to the treatment actually received.

Reported are the number of participants positive for ATAs at baseline, the number of participants with treatment-induced ATAs and the number of participants with treatment-enhanced ATAs.

Safety Population included all randomized participants who received study drug, with participants grouped according to the treatment actually received. Here, number analyzed are the participants who were evaluable for this outcome measure.

The time to normalization of respiratory function was defined as the time to removal of the participant from oxygen (O2) supplementation in order to maintain a blood oxygen saturation level (SpO2) equal to or greater than 95% as measured by pulse oximetry.

Intent-to-treat infected (ITTi) population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

The clinical status of participants was defined by five mutually exclusive categories: 1. Death; 2. In the Intensive Care Unit (ICU); 3. Non-ICU hospitalization, requiring supplemental oxygen (O2); 4. Non-ICU hospitalization, not requiring supplemental oxygen (O2); 5. Not hospitalized.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

Clinical failure after 24 hours post-infusion of study drug was defined as progression to increased O2 requirement defined by an increase in oxygen supplementation from low flow oxygen (i.e., 2-6 liters per minute [L/min]) to high flow oxygen (i.e., > 6 L/min) or from oxygen supplementation alone to any positive pressure ventilation (PPV) or extracorporeal membrane oxygenation (ECMO), progression to ICU, prolonged ventilation or O2 support defined by > 2 weeks, or death.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

Description: Clinical resolution of abnormal vital signs was defined as meeting three out of five of the following criteria: 1. SpO2 ≥ 95% without supplemental O2; 2. Respiratory rate < 24 breaths per minute without supplemental O2; 3. Core temperature < 37.2 Celsius (C) immediately prior to receipt of any antipyretic drug, and at least 6-8 hours from the last dose of antipyretic or core temperature > 36 C in participants who are initially hypothermic; 4. Heart rate (HR) < 100 beats/minute; 5. Systolic blood pressure (SBP) >90 mmHg. Reported here is the percentage of participants who had clinical resolution of at least three out of five abnormal vital signs by the end of study.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

Influenza A viral load was measured by quantitative polymerase chain reaction (qPCR) in nasopharyngeal samples at multiple time points during the study. AUEC is the area under the viral load-time curve expressed as log10 (viral particles/milliliter x hour) = log10 (vp/mL x hour).

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization. Here, number analyzed are the participants who were evaluable for this outcome measure.

Influenza A viral load was measured by qPCR in nasopharyngeal samples at multiple time points during the study. Reported here is the peak Influenza A viral load expressed as log10 vp/mL.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization. Here, number analyzed are the participants who were evaluable for this outcome measure.

Influenza A viral load was measured by qPCR in nasopharyngeal samples at multiple time points during the study. Reported here is the duration of viral shedding.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization. Here, number analyzed are the participants who were evaluable for this outcome measure.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization. Here, number analyzed are the participants who were evaluable for this outcome measure.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

The following were considered secondary complications of influenza: pneumonia, including hospital-acquired pneumonia (HAP) and ventilation-acquired pneumonia (VAP), exacerbations of chronic lung disease, myocarditis, acute respiratory distress syndrome (ARDS), otitis media, or other related complications.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization.

ITTi population included all randomized participants who were confirmed to be influenza A infected, with participants grouped according to the treatment assigned at randomization. Here, number analyzed are the participants who were evaluable for this outcome measure.

AUC0-inf is reported as day*microgram/milliliter (day*mcg/mL).

Pharmacokinetic (PK)-evaluable population included all participants, who received MHAA4549A and from whom evaluable PK samples were obtained.

PK-evaluable population included all participants, who received MHAA4549A and from whom evaluable PK samples were obtained.

PK-evaluable population included all participants, who received MHAA4549A and from whom evaluable PK samples were obtained.

PK-evaluable population included all participants, who received MHAA4549A and from whom evaluable PK samples were obtained.

PK-evaluable population included all participants, who received MHAA4549A and from whom evaluable PK samples were obtained.