A Study of MHAA4549A as Monotherapy for Acute Uncomplicated Seasonal Influenza A in Otherwise Healthy Adults

Inclusion Criteria: * Otherwise healthy participants * Positive test for influenza A infection * No more than 72 hours elapsed between onset of influenza-like illness and start of study drug * Presence of at least one moderate or severe constitutional symptom such as headache, myalgia, fever, chills, fatigue, anorexia, or nausea PLUS one moderate or severe respiratory symptom such as cough, sore throat, or rhinorrhea * For women of childbearing potential: negative pregnancy test and agreement to use acceptable contraceptive methods for at least 120 days after study drug administration * For men: agreement to use acceptable contraceptive methods for at least 30 days after study drug administration Exclusion Criteria: * Creatinine clearance less than or equal to (\</=) 80 milliliters per minute (mL/min) * Any significant medical conditions or laboratory abnormalities * Clinical signs and symptoms consistent with otitis, bronchitis, sinusitis, or pneumonia or active bacterial infection * Use of antiviral therapy in the period from onset of influenza-like illness and prior to enrollment * Pregnancy at Screening or is currently pregnant or breastfeeding * Investigational therapy within 30 days or 5 half-lives prior to start of study drug, whichever is greater * Prior anti-influenza monoclonal antibody use * Receipt of a nasal influenza A vaccine within 14 days prior to Screening * Positive test for influenza B or influenza A+B within 2 weeks prior to study drug * History of significant tobacco use or drug/alcohol abuse * Chronic use of oral or inhaled corticosteroids within 30 days prior to Screening * Autoimmune disease, known immunodeficiency of any cause, or use of immunosuppressive medications * History of any chronic respiratory condition * Human immunodeficiency virus (HIV) with cluster of differentiation (CD) 4 count \</= 200 cells per milliliter (cells/mL) in the past 12 months * Serious infection requiring oral or IV antibiotics within 14 days prior to Screening