Flu vs Influenza: Understanding the Terms
"Flu" and "influenza" are the same disease—there is no medical difference between these terms. "Flu" is simply the colloquial shorthand for influenza, an acute viral respiratory infection caused by influenza viruses 1, 2.
What is Influenza?
Influenza is an acute viral respiratory infection that affects all age groups and causes significant morbidity and mortality worldwide 1, 2. The disease is caused by three types of influenza viruses that infect humans:
- Influenza A is the most clinically significant type because it causes pandemics and has high susceptibility to antigenic variation, with currently circulating subtypes including H1N1 and H3N2 3, 2
- Influenza B circulates seasonally and causes epidemic disease but does not cause pandemics 3
- Influenza C causes mild respiratory illness and is less clinically significant 3
Nearly 10% of the world's population is affected by influenza annually, with approximately 500,000 deaths each year globally 1. In the United States alone, influenza causes more than 35,000 deaths and 200,000 hospitalizations annually 4.
Clinical Presentation
The hallmark of influenza infection is abrupt onset of specific symptoms 2:
- Fever (typically high-grade)
- Cough
- Chills or sweats
- Myalgias (muscle aches)
- Malaise
- Headache
- Sore throat
- Runny nose 3, 2
Most febrile illness lasts 3-4 days with complete resolution in 7-10 days 3. However, fever persisting beyond 6-7 days strongly suggests bacterial superinfection and requires immediate evaluation 5.
Diagnosis
For most outpatients, the diagnosis is made clinically during influenza season, and laboratory confirmation is not necessary 2. Clinical diagnosis is based on acute onset of fever with cough or sore throat during periods when influenza is circulating in the community 6.
Laboratory testing should be considered in specific situations 7, 2:
- Hospitalized patients with suspected influenza
- Patients for whom confirmed diagnosis will change treatment decisions
- When results will influence infection control measures 7
Diagnostic Testing Options
When testing is indicated, rapid molecular assays (RT-PCR) are the preferred diagnostic tests because they can be performed at point of care, are highly accurate, and provide fast results 2. Other available tests include 7:
- Viral culture (gold standard but slow)
- Rapid antigen testing (lower sensitivity, results in 30 minutes)
- Immunofluorescence
- Serology 7
Critical pitfall: Rapid antigen tests have poor sensitivity, and negative results should not exclude treatment in high-risk patients 6. Do not delay treatment while awaiting laboratory confirmation in high-risk patients, as delays reduce effectiveness 6.
Antiviral Treatment
Who Should Receive Treatment
All hospitalized patients with suspected or confirmed influenza should receive antiviral treatment immediately, regardless of symptom duration or vaccination status 6.
High-risk patients who should receive immediate treatment include 6:
- Children under 2 years of age (especially infants under 6 months)
- Adults 65 years and older
- Pregnant and postpartum women
- Immunocompromised patients (including those on long-term corticosteroids, chemotherapy, or with HIV)
- Patients with chronic medical conditions:
Timing of Antiviral Therapy
Oseltamivir should be initiated as soon as possible within 48 hours of symptom onset for maximum benefit 6. Treatment within this window reduces illness duration by approximately 1-1.5 days in otherwise healthy adults and 17.6-29.9 hours in children 6.
However, treatment beyond 48 hours still provides substantial mortality benefit in high-risk, severely ill, or hospitalized patients and should NOT be withheld 6. Multiple studies demonstrate significant mortality reduction (OR = 0.21 for death within 15 days) when treatment is initiated up to 96 hours after symptom onset in hospitalized patients 6, 5.
Oseltamivir Dosing
Adults and adolescents ≥13 years: 75 mg orally twice daily for 5 days 6
Pediatric weight-based dosing (twice daily for 5 days): 6
- ≤15 kg: 30 mg
15-23 kg: 45 mg
23-40 kg: 60 mg
40 kg: 75 mg
Renal adjustment: Reduce dose by 50% (75 mg once daily) if creatinine clearance <30 mL/min 6, 5
Expected Clinical Benefits
Oseltamivir treatment provides 6:
- Reduction in illness duration by 1-1.5 days when started within 48 hours
- 50% reduction in risk of pneumonia
- 34% reduction in otitis media in children
- Significant mortality benefit in hospitalized and high-risk patients (OR = 0.21)
- Reduced risk of hospitalization in outpatients
Common Adverse Effects
The most common side effect is vomiting, occurring in approximately 15% of treated children versus 9% on placebo 6. This is transient and rarely leads to discontinuation, and taking oseltamivir with food reduces nausea and vomiting 6. No established link between oseltamivir and neuropsychiatric events has been confirmed 6.
Antibiotic Management
Previously well adults with acute bronchitis complicating influenza, in the absence of pneumonia, do NOT routinely require antibiotics 7.
Antibiotics should be added when 7, 5:
- Worsening symptoms develop (recrudescent fever or increasing dyspnea)
- Pneumonia is confirmed on chest radiography
- Fever persists beyond 6-7 days (strongly suggests bacterial superinfection)
- Purulent sputum production develops
Antibiotic Regimens
For non-severe pneumonia or worsening symptoms without confirmed pneumonia: 7, 5
- First-line: Co-amoxiclav 625 mg PO three times daily OR doxycycline 100 mg PO once daily
- Alternative: Macrolide (clarithromycin or erythromycin) or fluoroquinolone active against S. pneumoniae and S. aureus
For severe influenza-related pneumonia: 7, 5
- IV co-amoxiclav OR second/third generation cephalosporin (cefuroxime or cefotaxime) PLUS macrolide (clarithromycin or erythromycin)
The most common bacterial pathogens causing secondary pneumonia after influenza are Staphylococcus aureus (including MRSA) and Streptococcus pneumoniae 5, 4.
Chemoprophylaxis
Vaccination remains the primary prevention strategy, and chemoprophylaxis is NOT a substitute for vaccination 7, 6.
Chemoprophylaxis with oseltamivir (75 mg once daily) should be considered for 7, 6:
- High-risk patients during the 2 weeks after vaccination (before adequate immune response develops)
- High-risk patients for whom vaccination is contraindicated, unavailable, or expected to have low effectiveness
- All residents of long-term care facilities during institutional outbreaks (regardless of vaccination status)
- Post-exposure prophylaxis for household contacts of influenza-infected persons, especially high-risk individuals, when started within 48 hours of exposure
Prophylaxis dosing: 75 mg once daily for 10 days after household exposure (adults and adolescents ≥13 years) 6. Pediatric dosing is weight-based at the same doses as treatment but given once daily 6.
Vaccination
Annual influenza vaccination is recommended for all people six months and older who do not have contraindications 2. Vaccination is the most effective method for preventing influenza infection and its complications 1. The vaccine has been clearly demonstrated to reduce serious morbidity and mortality, especially in high-risk groups 9.