This randomized, double-blind, placebo-controlled study will evaluate the efficacy and safety of MHAA4549A in healthy volunteers in an influenza challenge model. Subjects will be intranasally oculated with the challenge virus and will be randomly assigned to receive a intravenous dose of either MHAA4549A or placebo. In the stage 5 cohort, subjects will be randomly assigned to receive doses of MHAA4549A or placebo or Tamiflu (oseltamivir) orally twice daily for 5 days. All subjects in all cohorts may take Tamiflu twice daily from Day 7 to Day 11.

Inclusion Criteria:

• In good health with no history of major medical conditions from medical history, physical examination and routine laboratory tests
• Male subjects and women of child-bearing potential must use effective contraception as defined by protocol, starting at entry to quarantine and continuing until Day 120 follow-up visit
• Sero-suitable for challenge virus

Exclusion Criteria:

• Women who have been pregnant within 6 months prior to the study, or who have been breastfeeding within 3 months prior to the study, or who have a positive pregnancy test at any point in the study
• Any history or evidence of any clinically significant cardiovascular, gastrointestinal, endocrinological, haematological, hepatic, immunological, metabolic, urological, neurological, psychiatric, renal, and/or other major disease or malignancy, as judged by the CI
• Abnormal pulmonary function evidenced by clinically significant abnormalities in spirometry
• History of asthma, COPD, pulmonary hypertension, reactive airway disease, or chronic lung disease of any etiology. A history of childhood asthma before the age of 12 is acceptable provided the subject is asymptomatic without treatment.
• History suggestive of respiratory infection within 14 days prior to admission to the unit