A Study of MHAA4549A in Combination With Oseltamivir Versus Oseltamivir in Participants With Severe Influenza A Infection

Inclusion Criteria

• Diagnosis of influenza A where a Sponsor-approved influenza test is used as an aid in diagnosis. A Sponsor-approved influenza test includes: Influenza antigen test or Influenza polymerase chain reaction (PCR) test
• One of the following markers of severity within 24 hours of admission: requirement for O2 supplementation to maintain SpO2 greater than (>) 92 %; or requirement for Positive Pressure Ventilation (PPV)
• A negative urine or serum pregnancy test for women of childbearing potential within 2 days prior to study treatment
• Participants of reproductive potential must agree to use acceptable contraceptive measures as per the protocol as a minimum, and local guidelines, if more stringent

Exclusion Criteria

• Pregnant or lactating women, or women who intend to become pregnant during the study
• Hypersensitivity to monoclonal antibodies or any constituents (sodium succinate, sucrose, polysorbate 20) of study drug
• Hypersensitivity to the active substance or to any excipients of oseltamivir
• Investigational therapy within the 30 days prior to study treatment
• Received prior therapy with any anti-influenza monoclonal antibody therapy (including MHAA4549A) within 8 months prior to study treatment
• Current treatment (within 7 days of dosing) with probenecid, amantadine or rimantidine
• Participants who have taken more than a total of 6 doses (3 doses for peramivir) of anti-influenza therapy (e.g., oseltamivir, zanamivir, laninamivir, peramivir) in the period from onset of symptoms and prior to study treatment
• Admission >48 hours prior to study treatment
• Onset of influenza symptoms (including fever, chills, malaise, dry cough, loss of appetite, myalgias, coryza, or nausea) >5 days prior to study treatment
• Positive influenza B or influenza A + B infection within 2 weeks prior to study treatment
• High probability of mortality in the next 48 hours as determined by the investigator
• Participants requiring home or baseline oxygenation therapy
• Participants with history of chronic lung disease with a documented SpO2 less than (<) 95% off oxygen
• Participants on chronic dose of corticosteroids exceeding 10 milligrams per day (mg/day) of prednisone or equivalent steroid dose for duration of greater than 14 days within 30 days of entry into study
• Participants with the following significant immune suppression: bone marrow or solid organ transplant in the previous 12 months; cancer chemotherapy in the previous 12 months, HIV infection with most recent Cluster of Differentiation 4 (CD4) <200 cells per milliliter (cells/mL), or other significant immune suppression as determined by the investigator in discussion with the Sponsor Medical Monitor
• Participants on extracorporeal membrane oxygenation (ECMO) at time of randomization
• Any disease or condition that would, in the opinion of the site investigator or Sponsor, place the participant at an unacceptable risk of injury or render the participant unable to meet the requirements of the protocol