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