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

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.

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 (NEWS) 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