This double-blind (neither researchers nor participants know what treatment participant is receiving) study will evaluate efficacy/safety of pimodivir in combination with SOC treatment versus placebo in combination with SOC treatment in adolescent, adult, and elderly hospitalized participants with influenza A infection. The study will be conducted in 3 phases: screening phase, double-blind treatment period of 5 days (with the possibility to extend treatment period by 5 days for participants who will enter an optional double-blind extension treatment arm), and post treatment follow-up period of 23 days. Study evaluations will include efficacy, pharmacokinetic, biomarkers, safety and tolerability. The duration of participation in study for each participant is 28 days, except for participants receiving extended treatment, for whom study duration will be up to 33 days.
