What are the typical signs, complications, and management of influenza, especially in high‑risk groups?

Symptoms of Influenza

Influenza presents with abrupt onset of high fever (>38°C in adults, >38.5°C in children), myalgia, headache, severe malaise, and nonproductive cough—distinguishing it from the common cold by its sudden onset and predominance of systemic over respiratory symptoms. 1, 2

Core Clinical Features

Typical Presentation

• Fever: Temperature >38°C in adults or >38.5°C in children, appearing suddenly 3, 1
• Systemic symptoms predominate: Myalgia, severe malaise, headache, chills/sweating, and anorexia are more prominent than respiratory complaints 1, 4
• Respiratory symptoms: Nonproductive cough, sore throat, rhinitis, and nasal discharge develop alongside systemic features 3, 2
• Physical examination findings: Hot humid skin, flushed face, injected eyes, and hyperemic nasal/pharyngeal mucosae 1

Timing and Duration

• Incubation period: 1-4 days with an average of 2 days 3
• Infectious period: Patients are contagious from 1 day before symptom onset through approximately 5-6 days after illness begins 5, 4
• Children shed virus longer: Up to 10 days or more after symptom onset 3, 5
• Typical illness duration: 7 days, though cough and malaise can persist for 2 or more weeks 3, 1

Distinguishing Influenza from Common Cold

The combination of sudden onset plus high fever plus cough has a 79% positive predictive value for influenza when the virus is circulating in the community 1. Key differentiators include:

• Onset: Abrupt in influenza vs. gradual in common cold 1
• Fever: High-grade (>38°C) in influenza vs. absent or mild in cold 1
• Symptom predominance: Systemic symptoms dominate in influenza; nasal symptoms dominate in cold 1

Complications and Warning Signs

Respiratory Complications

• Primary viral pneumonia: Dyspnea within 48 hours of fever onset, productive cough with hemoptysis, cyanosis, tachypnea, bilateral crepitations 1
• Secondary bacterial pneumonia: Develops 4-5 days after onset during early convalescence, commonly caused by S. pneumoniae, S. aureus, and H. influenzae 3, 1
• Acute bronchitis: Cough, retrosternal discomfort, wheeze, and sputum production are integral to influenzal illness 3

Severe Illness Indicators Requiring Hospital Assessment

• Adults: Respiratory distress, cyanosis, oxygen saturation <90%, systolic blood pressure <90 mmHg, heart rate >100/min, respiratory rate >24/min, altered mental status, inability to maintain oral intake 6
• Children: Markedly raised respiratory rate, grunting, intercostal recession, cyanosis, severe dehydration, altered consciousness, complicated/prolonged seizures, signs of septicemia 3, 6

Other Organ System Involvement

• Cardiac: Myocarditis and pericarditis 7
• Neurologic: Confusion, convulsions, Guillain-Barré syndrome, encephalomyelitis, Reye's syndrome (in children given aspirin) 7
• Musculoskeletal: Myositis, myoglobinuria 7
• Renal: Acute renal failure 7

High-Risk Groups for Severe Disease

These populations have substantially elevated hospitalization and mortality rates:

• Age extremes: Adults ≥65 years (account for >90% of influenza deaths), children <5 years (especially infants <6 months with hospitalization rates ~1,040/100,000) 3, 6
• Chronic medical conditions: COPD, chronic heart disease, chronic renal disease, diabetes, immunosuppression 3, 6
• Pregnancy: All trimesters 6
• Immunocompromised hosts: May shed virus for weeks to months and experience prolonged symptoms 5, 4

Management Approach Based on Symptoms

Mild Symptoms (No High-Risk Features)

• Cough and mild fever: Treat at home with antipyretics (avoid aspirin in children) and fluids 3
• Antiviral consideration: If fever >38°C (adults) or >38.5°C (children) and presenting within 48 hours of symptom onset 3

Moderate Symptoms or High-Risk Patients

• Fever >38°C plus cough plus high-risk status: Requires medical evaluation 3
• Antiviral therapy: Oseltamivir 75 mg twice daily for 5 days in adults (reduce by 50% if creatinine clearance <30 mL/min) 6
• Antibiotic consideration: Not routinely required unless bacterial complications suspected (recrudescent fever, increasing breathlessness, focal chest signs) 3

Severe Symptoms Requiring Hospitalization

• Worsening respiratory symptoms: Particularly with bilateral chest signs, oxygen saturation <92%, or signs of pneumonia 6
• Combination therapy: Parenteral antibiotics covering S. aureus, S. pneumoniae, H. influenzae plus oseltamivir 3
• Preferred antibiotic regimen: Co-amoxiclav or second/third-generation cephalosporin plus macrolide (clarithromycin or erythromycin) 3

Common Pitfalls

• Delaying antiviral therapy: Greatest benefit occurs when started within 24 hours of symptom onset; do not wait for laboratory confirmation in high-risk patients 6, 2
• Inappropriate antibiotic use: Previously well adults with uncomplicated influenza or simple bronchitis do not require antibiotics 3
• Underestimating pediatric risk: Children <1 year and those with chronic conditions require lower threshold for medical evaluation and treatment 3
• Missing bacterial superinfection: Watch for recrudescent fever or worsening symptoms 4-5 days after initial improvement, suggesting secondary bacterial pneumonia 3, 1
• Aspirin in children: Never use aspirin in children with influenza due to Reye's syndrome risk 3