This is a multicenter, randomised, four arms placebo controlled study to evaluate the safety and tolerability of 10 mg inhaled zanamivir once a day and 75 mg oseltamivir capsule orally once a day relative to placebo over 16 weeks (112 days). Enrolled subjects will be randomly assigned to 2:1:2:1 ratio (active oseltamivir: placebo: active zanamivir: placebo) on one of the four study treatment groups.