Overview
Influenza (Flu)
1. Clinical Overview
Summary
Influenza is an acute viral respiratory infection caused by Influenza viruses A and B. It is a major cause of seasonal epidemics (Winter in temperate climates) and occasional pandemics (Novel strains). Influenza presents with abrupt onset of fever, myalgia, headache, sore throat, and dry cough. Unlike the "common cold," influenza is characterised by systemic symptoms (Feeling "wiped out") as well as respiratory symptoms. Most cases are self-limiting in healthy individuals, resolving within 1-2 weeks. However, influenza can cause severe illness and death in high-risk groups (Elderly, Pregnant, Immunocompromised, Chronic diseases). Complications include viral pneumonia, secondary bacterial pneumonia, and exacerbation of chronic conditions. Management is primarily supportive; antivirals (Oseltamivir, Zanamivir) are recommended for high-risk patients within 48 hours of symptom onset. Annual vaccination is the primary prevention strategy. [1,2,3]
Clinical Pearls
"Sudden Onset, Systemic Symptoms": Influenza hits suddenly. Fever, Myalgia, Fatigue predominate. Respiratory symptoms (Cough, Sore throat) follow.
"Flu ≠ Cold": Cold = Gradual onset, Runny nose, Sneezing. Flu = Abrupt onset, Fever, Myalgia, Prostration.
"48-Hour Window": Antivirals most effective if started within 48 hours of symptoms. Benefit diminishes after this.
"Vaccinate High-Risk Groups": Annual flu vaccine for Elderly, Pregnant, Chronic diseases, Healthcare workers.
2. Epidemiology
Demographics
| Factor | Notes |
|---|---|
| Seasonality | Winter (November-March in Northern Hemisphere). |
| Incidence | 5-15% of population affected annually. |
| Mortality | Significant. ~10,000-30,000 excess deaths per year in UK (Mainly elderly). |
Virus Types
| Type | Notes |
|---|---|
| Influenza A | Most common. Causes seasonal epidemics and pandemics. Subtypes by H (Haemagglutinin) and N (Neuraminidase): H1N1, H3N2. |
| Influenza B | Causes seasonal epidemics. Less severe. Two lineages (Victoria, Yamagata). |
| Influenza C | Mild illness. Not epidemiologically significant. |
High-Risk Groups
| Group | Notes |
|---|---|
| Elderly (≥65 years) | Highest mortality. Waning immunity. |
| Pregnant Women | Increased morbidity/mortality. Safe to vaccinate. |
| Chronic Conditions | Respiratory (Asthma, COPD), Cardiac, Renal, Hepatic, Diabetes, Immunosuppression. |
| Morbid Obesity | BMI ≥40. |
| Healthcare Workers | Occupational exposure. Should be vaccinated to protect patients. |
| Young Children (less than 5 years) | Higher hospitalisation rates. |
3. Pathophysiology
Virus Structure
- Single-stranded RNA virus (Orthomyxoviridae).
- Haemagglutinin (H): Surface glycoprotein. Binds to sialic acid receptors on respiratory epithelium. Target of neutralising antibodies.
- Neuraminidase (N): Surface glycoprotein. Cleaves sialic acid. Releases new virions. Target of Oseltamivir/Zanamivir.
Antigenic Variation
| Type | Mechanism | Consequence |
|---|---|---|
| Antigenic Drift | Point mutations in H and N genes. | Gradual changes. Annual vaccine reformulation needed. |
| Antigenic Shift | Reassortment of gene segments (Usually in pigs – "Mixing vessel"). | New subtype. No population immunity. PANDEMIC potential. |
Infection Cycle
- Entry: Virus inhaled. H protein binds to respiratory epithelium.
- Replication: Virus replicates in epithelial cells.
- Cytopathic Effect: Cell death. Inflammation. Loss of ciliated epithelium.
- Release: N protein cleaves sialic acid → New virions released.
- Immune Response: Systemic cytokine release → Fever, Myalgia, Fatigue.
4. Differential Diagnosis
| Condition | Key Features |
|---|---|
| Influenza | Abrupt onset, High fever, Myalgia, Dry cough, Prostration. Winter. |
| Common Cold (Rhinovirus) | Gradual onset, Runny nose, Sneezing, Mild symptoms, No fever/Myalgia. |
| COVID-19 | Overlapping symptoms. Loss of taste/smell (Less common now). PCR/LFT test to differentiate. |
| RSV (Respiratory Syncytial Virus) | Young children, Elderly. Bronchiolitis, Wheeze. |
| Streptococcal Pharyngitis | Sore throat predominates. Exudates. No cough. Centor criteria. |
| Bacterial Pneumonia | Productive cough, High fever, Focal chest signs, CXR consolidation. |
| Mycoplasma Pneumonia | Younger patients, Dry cough, Walking pneumonia, Extrapulmonary features. |
5. Clinical Presentation
Symptoms (Classic Presentation)
| Symptom | Notes |
|---|---|
| Sudden Onset | "I know exactly when it started." |
| Fever | High (38-40°C). May have rigors. |
| Myalgia | Severe muscle aches. "My body hurts everywhere." |
| Headache | Frontal, Retro-orbital. |
| Fatigue/Malaise | Profound. "Wiped out." May last weeks. |
| Dry Cough | Initially non-productive. May become productive. |
| Sore Throat | Common. |
| Rhinitis | Nasal congestion. Less prominent than in colds. |
| Anorexia | Loss of appetite. |
| GI Symptoms | Nausea, Vomiting, Diarrhoea (More common in children and with influenza B). |
Duration
| Phase | Timing |
|---|---|
| Incubation | 1-4 days (Average 2 days). |
| Acute Illness | 3-7 days. Fever and systemic symptoms resolve first. |
| Cough and Fatigue | May persist 2-4 weeks. |
Signs
| Sign | Notes |
|---|---|
| Fever | Often >38.5°C. |
| Flushed Face | |
| Injection of Conjunctivae | Red eyes. |
| Pharyngeal Erythema | Non-exudative. |
| Clear or Purulent Nasal Discharge | |
| Tachypnoea | If pneumonia developing. |
6. Investigations
Diagnosis
| Test | Notes |
|---|---|
| Clinical Diagnosis | Often sufficient during known influenza season. |
| Rapid Antigen Test (RAT) | Point-of-care. Quick (~15 min). Moderate sensitivity. |
| PCR (RT-PCR) | Gold standard. Nasal/Throat swab. Highly sensitive and specific. |
| Viral Culture | Research/Surveillance. Not routine clinical. |
When to Test
| Indication | Rationale |
|---|---|
| Hospitalised Patients | Confirm diagnosis. Inform infection control (Isolation). |
| High-Risk Patients | Guide antiviral therapy. |
| Severe/Atypical Illness | Differentiate from other pathogens. |
| Outbreaks | Confirm influenza. Public health surveillance. |
Other Investigations (If Unwell)
| Test | Rationale |
|---|---|
| CXR | If pneumonia suspected (Hypoxia, Focal signs). |
| FBC, U&Es, CRP | Assess severity. Secondary bacterial infection (Raised WCC, CRP). |
| ABG/VBG | If respiratory distress. |
7. Management
Management Algorithm
SUSPECTED INFLUENZA
(Winter, Abrupt onset, Fever, Myalgia, Cough)
↓
ASSESS SEVERITY
- Respiratory distress? SpO2?
- Dehydration?
- Altered mental status?
- High-risk group?
↓
SEVERITY ASSESSMENT
┌────────────────┴────────────────┐
MILD (Most Cases) SEVERE / HIGH-RISK
↓ ↓
OUTPATIENT MANAGEMENT CONSIDER ADMISSION
iv fluids, O2, monitoring
Supportive Care (All Patients)
| Intervention | Notes |
|---|---|
| Rest | Stay home. Avoid spreading. |
| Fluids | Maintain hydration. |
| Antipyretics/Analgesics | Paracetamol, Ibuprofen. For fever, Headache, Myalgia. |
| Avoid Aspirin in Children | Risk of Reye's Syndrome. |
Antiviral Therapy
| Drug | Dose | Notes |
|---|---|---|
| Oseltamivir (Tamiflu) | 75mg BD PO for 5 days | Neuraminidase inhibitor. FIRST-LINE. Start within 48 hours. |
| Zanamivir (Relenza) | 10mg (2 inhalations) BD for 5 days | Inhaled. Alternative if Oseltamivir resistant. Avoid in asthma/COPD (Bronchospasm risk). |
| Peramivir | IV option | For severe hospitalised patients unable to take oral/inhaled. |
| Baloxavir | Single dose oral | Endonuclease inhibitor. Newer. Not widely used UK. |
When to Give Antivirals?
| Indication | Notes |
|---|---|
| Hospitalised with Confirmed/Suspected Flu | All – regardless of symptom duration. |
| High-Risk Outpatients | Within 48 hours of symptom onset. Elderly, Pregnant, Chronic disease, Immunocompromised. |
| Severe/Progressive Illness | Even after 48 hours – some benefit possible. |
| Not Routinely for Healthy Adults | Self-limiting. Antivirals shorten illness by ~1 day. |
Infection Control (Hospital)
- Droplet Precautions: Surgical mask within 1 metre.
- Hand Hygiene: Alcohol gel.
- Isolation: Side room if possible.
- Staff: Vaccinated. PPE.
8. Complications
Respiratory
| Complication | Notes |
|---|---|
| Primary Viral Pneumonia | Severe, Rapid progression, ARDS. High mortality. |
| Secondary Bacterial Pneumonia | Follows initial improvement ("Biphasic illness"). S. pneumoniae, S. aureus, H. influenzae. |
| Acute Exacerbation of Asthma/COPD | Common. |
| Otitis Media | Especially children. |
| Sinusitis |
Non-Respiratory
| Complication | Notes |
|---|---|
| Myocarditis | Rare. Chest pain, Arrhythmia. |
| Encephalitis | Rare. Altered mental status. |
| Guillain-Barré Syndrome | Post-infectious. Rare. |
| Rhabdomyolysis | Rare. |
| Reye's Syndrome | Aspirin + Viral illness in children → Encephalopathy + Liver failure. |
9. Prognosis and Outcomes
| Factor | Notes |
|---|---|
| Healthy Adults | Self-limiting. Full recovery. |
| Duration | Acute: 3-7 days. Fatigue: 2-4 weeks. |
| High-Risk Groups | Increased risk of complications, Hospitalisation, Death. |
| Mortality | Low overall (~0.1%). Higher in elderly, Immunocompromised. |
10. Prevention
Vaccination
| Vaccine | Notes |
|---|---|
| Annual Flu Vaccine | Reformulated each year to match circulating strains. Live attenuated (Nasal – Children 2-17) or Inactivated (Injection – Adults). |
| Timing | Autumn (October-November) before flu season. |
| Eligible Groups (UK NHS) | ≥65 years, Pregnant, Chronic diseases (Asthma, Diabetes, Heart/Lung/Kidney disease), Immunocompromised, Healthcare workers, Carers, Children 2-16. |
| Effectiveness | Variable (40-60% depending on season/match). Reduces severity even if infected. |
Chemoprophylaxis
| Indication | Notes |
|---|---|
| Post-Exposure Prophylaxis | Oseltamivir 75mg OD for 10 days. For unvaccinated high-risk contacts during outbreak. |
| Outbreak in Care Home | All residents and staff may be offered prophylaxis. |
11. Evidence and Guidelines
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Flu Management | NICE NG158 (2022) | Antivirals for high-risk within 48h. Oseltamivir first-line. |
| Flu Immunisation | JCVI / PHE | Annual vaccination. Eligible groups. |
12. Patient and Layperson Explanation
What is Flu?
Flu (Influenza) is a viral infection that affects the respiratory system (Nose, Throat, Lungs). It is NOT the same as a common cold. Flu usually makes you feel much worse – with sudden high fever, severe body aches, and extreme tiredness.
How is it different from a cold?
- Cold: Comes on gradually. Runny nose, Sneezing. Mild tiredness.
- Flu: Comes on suddenly. High fever, Severe muscle aches, Exhaustion. You feel "knocked out."
Is Flu serious?
For most healthy people, flu is unpleasant but not dangerous. You will feel awful for a few days, then gradually recover. However, flu can be serious for:
- Older people (65+)
- Pregnant women
- People with chronic diseases (Asthma, Diabetes, Heart disease)
- People with weakened immune systems
What is the treatment?
- Rest and Fluids – Stay home and drink plenty.
- Paracetamol/Ibuprofen – For fever and aches.
- Antiviral tablets (Tamiflu) – Sometimes prescribed for high-risk people if caught early.
Should I get the flu vaccine?
If you are in a high-risk group, the NHS offers a free annual flu vaccine. It significantly reduces your risk of getting flu and makes illness milder if you do catch it.
13. References
Primary Sources
- National Institute for Health and Care Excellence. Influenza – seasonal (NG158). 2022. nice.org.uk/guidance/ng158
- Uyeki TM, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2019;68(6):e1-e47. PMID: 30566567.
- Public Health England. Influenza: the green book, chapter 19. 2021.
14. Examination Focus
Common Exam Questions
- Differentiate Flu from Cold: "How do you distinguish Influenza from Common Cold?"
- Answer: Flu = Sudden onset, High fever, Severe myalgia, Prostration. Cold = Gradual, Rhinitis, Mild symptoms.
- Antivirals: "When should Oseltamivir be given?"
- Answer: Within 48 hours of symptoms in High-risk patients or Hospitalised patients.
- Antigenic Shift vs Drift: "What is the difference?"
- Answer: Drift = Point mutations → Annual vaccine updates. Shift = Gene reassortment → New subtype → Pandemic.
- High-Risk Groups: "Who should receive the flu vaccine?"
- Answer: ≥65 years, Pregnant, Chronic diseases, Immunocompromised, Healthcare workers.
Viva Points
- Reye's Syndrome: Avoid Aspirin in children with viral illness.
- 48-Hour Window: Antivirals less effective after this.
- Secondary Bacterial Pneumonia: "Biphasic illness" – Improvement then deterioration. S. aureus, S. pneumoniae.
- Pandemic Influenza: Antigenic shift (1918 H1N1, 2009 H1N1).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.