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

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