Long Term Influenza Prophylaxis With Inhaled Zanamivir or Oral Oseltamivir

Inclusion Criteria: * A subject will be eligible for inclusion in this study only if all of the following criteria apply: 1. Healthy male or female aged 20 - 65 years. Healthy determined by the Investigator based on medical history and screening examinations. 2. Read and understand study related materials intended for subject. 3. Signed informed consent prior to study participation. 4. Willingness and ability to comply with the study protocol for the duration of the trial. 5. Female subject who is of reproductive potential agrees to use an acceptable method of birth control (e.g., intrauterine device, abstinence or hormonal contraceptive drug) throughout the study (D120+2). 6. For male subject, he must agree to use an acceptable method of birth control (e.g. abstinence or condom) throughout the study (D120+2). Exclusion Criteria: * A subject will not be eligible for inclusion in this study if any of the following apply: 1. Females who are pregnant, trying to get pregnant or are lactating. 2. Current or a history of asthma or chronic obstructive pulmonary disease within the last 5 years. 3. Abnormal pulmonary function test according to The Thoracic Society of Thailand Guidelines for Pulmonary Function Tests: (i) FVC \< 80% of the predicted value for age and sex or (ii) FEV1/FVC \<70%. (see Appendix 2) 4. Any other chronic or acute diseases requiring treatment. 5. An abnormal ECG indicating disease that requires immediate investigation or treatment 6. An abnormal chest x-ray indicating an active disease process. 7. A creatinine clearance \< 30 mL/min as determined by Cockcroft-Gault equation (see below) 8. AST or ALT ≥ 1.5 x ULN 9. Use of a live attenuated influenza vaccine or any anti-influenza drug within 14 days prior to the first dose of study drugs/placebos. 10. History of alcohol or substance abuse or dependence which in the opinion of the investigator would interfere with subject compliance or safety 11. Subjects who have a history of allergy to the study drug or drugs of the same class, or a history of severe drug (e.g. toxic epidermal necrolysis) or other allergy (e.g. previous history of anaphylaxis) that, in the opinion of the investigator, contraindicates participation in the trial. 12. The subject has participated in a clinical trial and has received investigational drugs within 60 days prior to the first dose of study drug. 13. Subjects who, in the opinion of the investigator, are unlikely to comply fully with study procedures. 14. History of any mental illness requiring hospitalisation or outpatient psychiatric treatment within the last 12 months 15. Any suicidal attempt within the previous 5 years. 16. Screening HADS anxiety or depression score \> 8. 17. Hemoglobin \<11g/dL or neutropenia of \< 1000 per microlitre or a platelet count of less than 100,000 cells/mm3 Cockroft formula: CrCl (mL/min) = (140 - age) x Wt / 72 x serum creatinine (females x 0.85), age in years, Wt in kg, serum creatinine in mg/dL.