The purpose of this study is to evaluate the clinical and virologic benefit of pimodivir in combination with Standard-of-Care (SOC) treatment compared to placebo in combination with SOC treatment.

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 (13 to 17 years), adult (18 to 65 years), and elderly (greater than \[\>\] 65 but less than or equal to \[\<=\] 85 years) non-hospitalized participants with influenza A infection who are at risk of developing complications. The study will be conducted in 3 phases: screening phase, double-blind treatment period (5 days), a post treatment follow-up period (23 days). Study evaluations include efficacy, clinical and virological outcomes, pharmacokinetics (PK), PK/pharmacodynamics, biomarkers, safety and tolerability. The duration of participation in the study for each participant is 28 days.