Summary

Influenza is an acute respiratory infection caused by influenza viruses (types A, B, and rarely C). It causes seasonal epidemics and occasional pandemics, resulting in significant morbidity and mortality worldwide. Influenza A is responsible for most epidemics and all pandemics due to antigenic variation. Clinical presentation includes abrupt onset of fever, myalgia, headache, and respiratory symptoms. Most cases are self-limiting, but complications including viral pneumonia and secondary bacterial infection can be fatal. Antivirals (oseltamivir) are effective if started within 48 hours of symptom onset, particularly in high-risk groups. Annual vaccination is the cornerstone of prevention. [1,2]

Key Facts

• Incidence: 5-10% of adults, 20-30% of children annually during seasonal epidemics. [3]
• Mortality: 290,000-650,000 deaths globally per year.
• Causative Agents: Influenza A (most common, pandemics), Influenza B (seasonal), Influenza C (rare, mild).
• Classification: A subtypes by Haemagglutinin (H1-18) and Neuraminidase (N1-11); e.g., H1N1, H3N2.
• Transmission: Respiratory droplets; incubation 1-4 days.
• Treatment: Supportive care; antivirals (oseltamivir) within 48 hours for high-risk.
• Prevention: Annual influenza vaccination.

Clinical Pearls

Antigenic Drift vs Shift: Drift = minor mutations causing seasonal variants (reason for annual vaccine). Shift = major reassortment creating novel subtypes (pandemic potential, e.g., 2009 H1N1 "Swine Flu").

The 48-Hour Window: Antivirals are most effective if started within 48 hours of symptom onset. After 48 hours, benefit is reduced but may still help in severe disease or high-risk patients.

Secondary Bacterial Pneumonia: Consider if patient improves then deteriorates ("biphasic illness"). Staphylococcus aureus (including MRSA) and Streptococcus pneumoniae are common culprits.

Flu vs Cold: Influenza = abrupt onset, high fever, severe myalgia, prostration. Common cold = gradual onset, milder, predominantly nasal symptoms.

General Assessment

• Fever, flushed appearance.
• Lethargy, prostration.
• Dehydration (reduced intake, fever).

Respiratory Examination

Inspection

• Respiratory rate (tachypnoea if pneumonia/ARDS).
• Accessory muscle use.
• Cyanosis (severe hypoxia).

Auscultation

• Uncomplicated: Often clear or scattered wheeze.
• Viral pneumonia: Bilateral crackles.
• Secondary bacterial: Focal consolidation.

Oxygen Saturation

• SpO₂ less than 92% on room air = concerning.

Signs of Complications

Uncomplicated Influenza

• Duration: 5-7 days for acute symptoms; fatigue/cough may persist 2+ weeks.
• Full Recovery: Expected in healthy individuals.

Hospitalised Patients

• Mortality: 5-10% in hospitalised patients; higher in ICU.
• Length of Stay: 5-7 days on average.
• ICU Admission: 10-20% of hospitalised cases.

Mortality

Long-Term Outcomes

• Post-Viral Fatigue: May persist weeks.
• Post-Influenza Complications: Cardiovascular events increased in weeks after flu.
• Neurological Sequelae: Rare (post-encephalitic).

Key Guidelines

Landmark Studies

1. Dobson et al. Cochrane Review (2015) [4]

• Question: What is the efficacy of oseltamivir and zanamivir?
• Result: Oseltamivir reduces symptom duration by about 1 day; reduces complications in at-risk.
• Impact: Supports targeted antiviral use in high-risk groups.
• Clinical Trial: Systematic review of 53 randomized controlled trials.
• PMID: 25865455.

2. PRIDE Consortium Study (2015)

• Question: Does oseltamivir reduce mortality in hospitalised patients?
• N: Meta-analysis of 20,000+ patients across multiple trials.
• Result: Early oseltamivir associated with reduced mortality (OR 0.81).
• Impact: Supports early treatment in hospitalised patients regardless of duration.
• Clinical Trial: International observational study consortium.
• PMID: 25559095.

3. Hayden et al. Oseltamivir Prevention RCT (1999) [6]

• Question: Does oseltamivir prevent experimental influenza infection?
• N: 117 healthy volunteers, double-blind randomized controlled trial.
• Result: Oseltamivir 100mg BD reduced infection rate by 82% (pless than 0.001).
• Impact: First demonstration of neuraminidase inhibitor prophylactic efficacy.
• Clinical Trial: Experimental human infection model study.
• PMID: 10517425.

4. Treanor et al. Oseltamivir Treatment RCT (2000) [7]

• Question: Does oseltamivir reduce duration of naturally acquired influenza?
• N: 629 patients, multinational randomized controlled trial.
• Result: Reduced median symptom duration by 1.3 days (p=0.03).
• Impact: Established oseltamivir as first-line treatment for influenza.
• Clinical Trial: Phase III randomized trial.
• PMID: 10697061.

5. Nicholson et al. Oseltamivir European RCT (2000) [8]

• Question: Oseltamivir efficacy in European influenza patients.
• N: 726 patients across 6 countries, randomized controlled trial.
• Result: 38% reduction in symptom severity; reduced antibiotic use by 26%.
• Impact: Confirmed efficacy across different influenza strains and populations.
• Clinical Trial: European multicenter randomized trial.
• PMID: 10866439.

What is Influenza (Flu)?

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and is different from a common cold. Flu season typically occurs in autumn and winter.

How is it Different from a Cold?

• Flu: Sudden onset, high fever, severe muscle aches, extreme tiredness.
• Cold: Gradual onset, low fever, mainly runny nose and sneezing.

Symptoms of Flu

• High fever and chills.
• Severe body aches and headache.
• Extreme tiredness.
• Dry cough.
• Sore throat.
• Runny or blocked nose.

Most people recover in 1-2 weeks, but fatigue may last longer.

Who is at Risk of Complications?

• Older adults (65+).
• Children under 5.
• Pregnant women.
• People with chronic illnesses (heart, lung, kidney, diabetes).
• People with weakened immune systems.

When to See a Doctor

• Difficulty breathing or shortness of breath.
• Persistent chest pain.
• Confusion or altered consciousness.
• Severe vomiting.
• Symptoms that improve then worsen.
• Very high fever that doesn't respond to medication.

How is Flu Treated?

Most People: Rest, fluids, paracetamol for fever. Antivirals (Tamiflu): May be prescribed for high-risk groups, especially if started within 48 hours of symptoms.

Can Flu be Prevented?

Yes!

• Annual Flu Vaccine: Best protection; recommended for high-risk groups and healthcare workers.
• Hand Hygiene: Wash hands frequently.
• Cover Coughs/Sneezes: Use tissues or elbow.
• Stay Home if Ill: Prevents spreading to others.

Why Get the Flu Vaccine?

• Reduces risk of getting flu.
• Reduces severity if you do get flu.
• Protects vulnerable people around you.
• Updated each year to match circulating strains.

(50-80 lines minimum - NEW MANDATORY SECTION)

Common Exam Questions

Questions that frequently appear in examinations:

1. MRCP: "A 45-year-old man presents with sudden onset fever, myalgia, and dry cough. How would you manage suspected influenza?"
2. USMLE: "Describe the differences between influenza A, B, and C viruses"
3. FRCR: "What are the indications for antiviral therapy in influenza?"
4. MRCS: "Explain the mechanism of action of oseltamivir"
5. Final MBBS: "What are the complications of influenza infection?"
6. PLAB: "A pregnant woman presents with flu-like symptoms. What is your management?"
7. USMLE: "How does antigenic shift differ from antigenic drift in influenza viruses?"
8. MRCP: "What is the role of vaccination in influenza prevention?"
9. Final MBBS: "Describe the transmission and incubation period of influenza"
10. USMLE: "What are the red flags for severe influenza requiring hospitalisation?"

Viva Points

Opening Statement (How to start your viva answer):

"Influenza is an acute viral respiratory infection caused by influenza viruses types A, B, and C. It presents with abrupt onset fever, myalgia, headache, and respiratory symptoms. The most important aspect is identifying high-risk patients who benefit from antiviral therapy, particularly within 48 hours of symptom onset. Vaccination remains the cornerstone of prevention."

Key Facts to Mention:

• Epidemiology: Seasonal epidemics November-March (Northern hemisphere), global mortality 290,000-650,000 annually
• Virology: Type A (pandemics, subtypes H1N1/H3N2), Type B (seasonal), Type C (mild)
• Transmission: Respiratory droplets, incubation 1-4 days, infectious 1 day before to 5-7 days after symptoms
• Clinical features: Abrupt onset, high fever, severe myalgia, dry cough - distinguishes from common cold
• Diagnosis: Clinical during epidemics; RT-PCR gold standard for confirmation
• Treatment: Supportive care; oseltamivir 75mg BD x 5 days within 48 hours for high-risk
• Prevention: Annual vaccination (40-60% efficacy), hand hygiene, respiratory etiquette

Classification to Quote:

• "Influenza viruses are classified into types A, B, and C based on nucleoprotein and matrix protein differences"
• "Influenza A subtypes are designated by haemagglutinin (H1-18) and neuraminidase (N1-11) surface proteins"
• "WHO classifies influenza severity as mild (outpatient), moderate (hospitalisation), severe (ICU/respiratory failure)"

Evidence to Cite:

• "The Cochrane review (2014) shows oseltamivir reduces symptom duration by approximately 1 day in adults"
• "PRIDE study (2015) demonstrated early oseltamivir use reduces mortality in hospitalised influenza patients"
• "CDC data shows influenza vaccination reduces severe outcomes by 40-60% even when imperfectly matched to circulating strains"
• "Jefferson et al. meta-analysis confirms antivirals reduce complications in high-risk groups when started within 48 hours"

Structured Answer Framework:

1. Definition and Epidemiology (30 seconds)

   • Acute viral infection by influenza A/B/C
   • Seasonal epidemics, global mortality 290,000-650,000 annually
   • Higher in elderly (>65 years), children (less than 5 years), immunocompromised

2. Pathophysiology (30 seconds)

   • Viral attachment via HA to sialic acid receptors
   • Replication in respiratory epithelium
   • Host inflammatory response causes symptoms
   • Antigenic drift/shift explains epidemics/pandemics

3. Clinical Features (45 seconds)

   • Abrupt onset fever, chills, myalgia, headache
   • Dry cough, sore throat, malaise
   • Distinguish from common cold by severity and systemic symptoms
   • Red flags: hypoxia, confusion, biphasic illness (secondary bacterial pneumonia)

4. Investigations (30 seconds)

   • Clinical diagnosis during epidemics
   • RT-PCR nasopharyngeal swab for confirmation
   • CXR if pneumonia suspected
   • Bloods for complications (FBC, CRP, blood cultures)

5. Management (60 seconds)

   • Supportive: rest, fluids, antipyretics
   • Antivirals: oseltamivir 75mg BD x 5 days within 48 hours for high-risk
   • Antibiotics if secondary bacterial infection
   • Hospitalisation: hypoxia, severe disease, high-risk groups

6. Prevention and Complications (30 seconds)

   • Annual vaccination cornerstone
   • Complications: viral/bacterial pneumonia, ARDS, myocarditis, encephalitis
   • Prevention: hand hygiene, respiratory etiquette, isolation

Common Mistakes

What fails candidates:

• ❌ Confusing influenza with common cold (missing severity of symptoms)
• ❌ Not recognising 48-hour window for antivirals
• ❌ Forgetting vaccination as primary prevention
• ❌ Missing secondary bacterial pneumonia in biphasic illness
• ❌ Not identifying high-risk groups for treatment

Dangerous Errors to Avoid:

• ⚠️ Delaying antiviral therapy in high-risk patients
• ⚠️ Prescribing aspirin to children (Reye's syndrome risk)
• ⚠️ Missing bacterial superinfection requiring antibiotics
• ⚠️ Not hospitalising patients with respiratory distress

Outdated Practices (Do NOT mention):

• Routine antibiotics for uncomplicated influenza
• Amantadine/rimantadine (obsolete due to resistance)
• Whole-virus vaccines (replaced by subunit/split vaccines)

Examiner Follow-Up Questions

Expect these follow-up questions:

1. "What is the mechanism of antiviral resistance in influenza?"

   • Answer: "Resistance occurs through mutations in neuraminidase gene (H275Y in N1 subtype) or HA gene, reducing drug binding affinity"

2. "How do you counsel patients about influenza vaccination?"

   • Answer: "Explain 40-60% efficacy, protection against severe disease even with mismatch, safety in most groups, annual requirement due to antigenic change"

3. "What are the contraindications to oseltamivir?"

   • Answer: "Hypersensitivity to drug, caution in pregnancy (FDA category C), monitor for neuropsychiatric effects especially in children"

4. "Describe the public health measures for influenza control?"

   • Answer: "Surveillance systems, vaccine strain selection, antiviral stockpiles, infection control measures, pandemic preparedness plans"

Last Reviewed: 2025-12-24 | MedVellum Editorial Team

Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.