Infectious Disease

Influenza

An acute respiratory viral infection whose rapid antigenic change allows it to repeatedly evade population immunity, producing seasonal epidemics and occasional pandemics.

In a nutshell

Influenza is usually self-limiting, but older age, pregnancy, chronic disease, immunosuppression, hospitalisation and severe/progressive illness increase complication risk. Treat high-risk or severe disease promptly with current UKHSA antivirals guidance, assess pneumonia/sepsis, and prevent disease through seasonal vaccination.

Classic presentation

Abrupt fever, headache, myalgia, cough and marked malaise, often with respiratory symptoms; severe disease produces hypoxia, pneumonia, dehydration or sepsis.

Key points

PCR is mainly for hospitalised, severe or high-risk cases when it changes management or infection control.
Start antivirals promptly in high-risk, hospitalised or severe/progressive illness; do not wait for a test if treatment is indicated.
Late antiviral treatment can still be appropriate in severe or high-risk disease.
Antibiotics are for suspected bacterial complications, not uncomplicated influenza.
Vaccination eligibility and product choice are season-specific UKHSA policy.

First-line investigation

Clinical assessment with observations and oxygen saturation; use PCR, chest imaging and blood tests selectively according to severity and setting.

Management

Assess severity and risk

Use observations, oxygen saturation, hydration, comorbidity, pregnancy, age and illness trajectory to separate uncomplicated from severe/progressive influenza.^1,3

Support uncomplicated illness

Give fluids, rest, symptom relief and clear safety-netting; avoid routine antibiotics when there is no bacterial complication.^3,1

Treat high-risk or severe disease

Start current UKHSA-guided antiviral treatment promptly; check BNF for agent, renal function, interactions, pregnancy and paediatric advice.^1,5

Treat complications

Escalate hypoxia, respiratory distress, sepsis, dehydration, altered consciousness or suspected bacterial pneumonia.^1,3,5

Prevent future infection

Offer the current season’s vaccine to eligible groups and follow current product, contraindication and infection-prevention guidance.^4,6,2

Exam traps

A positive influenza test does not exclude bacterial pneumonia.
A normal oxygen saturation at rest does not remove the need to safety-net deteriorating patients.
Antivirals are not restricted to the first 48 hours when illness is severe or the patient is high risk.
Check the current seasonal programme before stating vaccine eligibility or product details.

Illustrations

Antigenic drift versus shiftDiagram comparing gradual haemagglutinin and neuraminidase mutation (drift) with genome reassortment producing a novel pandemic strain (shift).PassFinals · original

Influenza versus common cold symptom onsetComparison chart of abrupt systemic influenza symptoms against gradual, predominantly nasal cold symptoms.PassFinals · original

Chest X-ray in secondary bacterial pneumoniaChest radiograph showing focal consolidation complicating influenza infection.Mikael HÃ¤ggstrÃ¶m, M.D. Author info, Wikimedia Commons · CC0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.