A Study to Evaluate the Efficacy and Safety of Pimodivir in Combination With the Standard-of-Care Treatment in Adolescent, Adult, and Elderly Non-Hospitalized Participants With Influenza A Infection Who Are at Risk of Developing Complications

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.

Eligibility Criteria:
Inclusion Criteria:

* Present to the clinic with symptoms suggestive of a diagnosis of acute influenza and have at least 1 respiratory symptom and at least 1 systemic symptom, both scored as at least "moderate" if the symptom did not pre-exist before influenza onset, or scored worse than usual if the symptom pre-existed as determined by subject's ratings on Module 1 of the Flu-iiQ and the Pre-existing Symptom Questionnaire in the ePRO device. Symptoms must include the following by category: a) Respiratory symptoms: cough, sore throat, nasal congestion b) Systemic symptoms: headache, body aches or pain, feverishness, fatigue
* Tested positive for influenza A infection after the onset of symptoms, using a rapid influenza diagnostic test (RIDT) or, if available, a polymerase chain reaction (PCR)-based or other rapid molecular diagnostic assay
* Not be in need of hospitalized medical care at screening. Emergency room or hospital observation status for an anticipated duration of less than (\<)24 hours is not considered hospitalization as long as a determination of the need for hospitalization has not been made
* Enrollment and initiation of study drug treatment less than or equal to (\<=)72 hours after onset of influenza symptoms
* Participants 13 to 65 years of age, inclusive must also have at least 1 of the following: a) Cardiovascular or cerebrovascular disease (including congenital heart disease, chronic heart failure, coronary artery disease, or stroke; excluding isolated hypertension); b) Chronic lung disease (for example, asthma, chronic obstructive lung disease \[COPD\] or cystic fibrosis); c) Weakened immune system due to disease or medication (for example, participants with human immunodeficiency virus \[HIV\], cancer, or chronic liver or kidney disease \[presence of kidney damage for \>3 months, defined by structural or functional abnormalities of the kidney, with or without decreased GFR manifested by: pathological abnormalities; OR markers of kidney damage, including abnormalities in the composition of the blood or urine or abnormalities in imaging tests\], or participants taking chronic systemic steroids)

Exclusion Criteria:

* Received more than (\>)1 dose of influenza antiviral medication (for example, oseltamivir \[OST\] or zanamivir), or any dose of ribavirin within 2 weeks, prior to first study drug intake, or received intravenous (IV) peramivir \>1 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, history of a lung transplant)