A Study to Evaluate the Efficacy and Safety of Pimodivir in Combination With the Standard-of-Care Treatment in Adolescent, Adult, and Elderly Hospitalized Participants With Influenza A Infection

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, 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.

Eligibility Criteria:
Inclusion Criteria:

* Tested positive for influenza A infection after the onset of symptoms using a polymerase chain reaction (PCR)-based or other rapid molecular diagnostic assay
* Requires hospitalization to treat influenza infection and/or to treat complications of influenza infection (for example, radiological signs of lower respiratory tract disease, septic shock, central nervous system \[CNS\] involvement, myositis, rhabdomyolysis, acute exacerbation of chronic kidney disease, severe dehydration, myocarditis, pericarditis, ischemic heart disease, exacerbation of underlying chronic pulmonary disease, including asthma, chronic obstructive pulmonary disease \[COPD\], decompensation of previously controlled diabetes mellitus), including participants admitted to the Intensive Care Unit (ICU)
* Enrollment and initiation of study drug treatment less than or equal to (\<=)96 hours after onset of influenza symptoms
* Being on invasive mechanical ventilation or having a peripheral capillary oxygen saturation (SpO2) less than (\<)94 percent (%) on room air during screening. Participants with known pre-influenza SpO2 \<94% must have an SpO2 decline greater than or equal to (\>=)3% from pre-influenza SpO2 during screening
* Having a screening/baseline National Early Warning Score 2 (NEWS2) of \>=4

Exclusion Criteria:

* Received more than 3 doses of influenza antiviral medication (for example, oseltamivir \[OST\] or zanamivir), or any dose of ribavarin (RBV) within 2 weeks, prior to first study drug intake. Received intravenous (IV) peramivir more than one day prior to screening
* Unstable angina pectoris or myocardial infarction within 30 days prior to screening (inclusive)
* Presence of clinically significant heart arrhythmias, uncontrolled, unstable atrial arrhythmia, or sustained ventricular arrhythmia, or risk factors for Torsade de Pointes syndrome
* Known severe hepatic impairment (Child Pugh C cirrhosis) or chronic hepatitis C infection undergoing hepatitis C antiviral therapy
* Severely immunocompromised in the opinion of the investigator (for example, known cluster of differentiation 4 plus \[CD4+\] count \<200 cells per cubic millimeter \[cells/mm\^3\], absolute neutrophil count \<750/mm\^3, first course of chemotherapy completed within 2 weeks prior to screening, history of stem cell transplant within 1 year prior to screening, any history of a lung transplant)
* Known allergies, hypersensitivity, or intolerance to pimodivir or its excipients