What are the symptoms and treatment options for influenza in adults and children?

Symptoms of Influenza

Influenza typically presents as an acute febrile respiratory illness with abrupt onset of fever, cough, chills or sweats, myalgias, and malaise. 1

Classic Presentation in Adults and Adolescents

The hallmark symptoms include:

• Fever with acute onset of respiratory symptoms (cough, sore throat, nasal congestion) occurring together 2
• Myalgias (muscle aches) and malaise are characteristic features that help distinguish influenza from other respiratory infections 1
• Chills or sweats accompanying the febrile illness 1
• Headache is commonly reported 3
• Symptoms typically last 3-4 days with complete resolution in 7-10 days 3

Atypical Presentations Requiring High Clinical Suspicion

Elderly patients may present without fever but show new or worsening respiratory symptoms, exacerbation of congestive heart failure, or changes in mental status 2. This is a critical pitfall—do not wait for fever in older adults.

Infants and young children may present with:

• Fever without other localizing signs or symptoms 2
• Fever lasting 3-4 days as the primary manifestation 3

Immunocompromised patients warrant special attention:

• May have blunted or absent fever despite severe infection 4
• Can present with atypical or less characteristic manifestations 4
• Any fever >37.8°C (100°F) with respiratory symptoms requires urgent evaluation 4

Severe or Complicated Influenza

Consider these presentations that indicate potential complications:

• Acute exacerbation of chronic lung disease with fever 2
• Severe clinical presentation with fever or hypothermia 2
• Respiratory distress or shortness of breath indicating potential progression to pneumonia or respiratory failure 4
• Persistent fever beyond 4-5 days suggesting bacterial superinfection 4
• Recrudescent fever (fever returning after initial improvement) strongly suggests secondary bacterial pneumonia 4

Treatment Considerations Based on Symptoms

Antiviral therapy should be initiated within 48 hours of symptom onset for maximum benefit, reducing illness duration by approximately 24 hours 1. The FDA-approved dosing for oseltamivir is 75 mg twice daily for 5 days in adults and adolescents ≥13 years 5.

However, immunocompromised patients benefit from antiviral treatment even beyond 48 hours of symptom onset, as they can shed virus for weeks to months 4. This represents a critical distinction from immunocompetent patients.

All hospitalized patients with influenza should receive antiviral therapy regardless of time since symptom onset 2.

Key Clinical Pitfalls to Avoid

• Do not dismiss absence of fever in elderly or immunocompromised patients—they may have severe infection without typical febrile response 2, 4
• Do not assume symptoms beyond 48 hours are "too late" for treatment in high-risk patients, particularly those hospitalized or immunocompromised 4
• Do not overlook non-specific symptoms during flu season such as isolated cough or vague respiratory complaints in immunocompromised patients 4
• Clinical diagnosis alone is sufficient in outpatient settings during influenza season when typical symptoms are present; laboratory confirmation is not necessary for most patients 1

Epidemiologic Context Matters

During influenza season (when viruses circulate in the community), maintain high suspicion for influenza in:

• Anyone with fever and acute respiratory symptoms, regardless of vaccination status 2
• Healthcare workers, nursing home residents, or close contacts of confirmed cases with febrile respiratory illness 2
• Travelers returning from areas with circulating influenza presenting with compatible symptoms 2