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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Influenza A (subtypes H and N — pandemic potential) and B (seasonal). Transmitted by respiratory droplets. Incubation 1-4 days, infectious from 1 day before to 5-7 days after symptom onset
- Classic presentation: sudden onset high fever, severe myalgia, headache, dry cough, sore throat, malaise — distinguishes from common cold by systemic severity
- Diagnosis: clinical in most cases. Confirm with nasopharyngeal swab for PCR (in hospitalised or high-risk patients, or during surveillance)
- Treatment: oseltamivir 75 mg BD for 5 days — most effective if started within 48 hours of symptom onset. Indicated for at-risk groups and hospitalised patients
- Annual vaccination recommended for: over 65s, pregnant women, chronic disease (asthma, COPD, diabetes, CKD, immunosuppressed), healthcare workers, carers
Overview
Influenza is an acute viral respiratory infection caused by influenza A or B viruses. Influenza A has greater pandemic potential due to antigenic shift (reassortment of genome segments between human and animal strains). Seasonal epidemics occur annually, typically peaking in winter. Most cases are self-limiting, but complications (viral pneumonia, secondary bacterial pneumonia, exacerbation of chronic disease) cause significant morbidity and mortality, particularly in high-risk groups.
Epidemiology
Influenza causes an estimated 8,000-11,000 excess deaths per year in England during an average season, rising to 20,000+ in severe seasons. The elderly and those with chronic conditions bear the highest burden. The UK flu vaccination programme is one of the largest in the world, targeting high-risk groups and healthcare workers. Notable pandemics include H1N1 (2009 "swine flu"), H2N2 (1957), and H3N2 (1968).
Clinical Features
Symptoms
Sudden onset high fever (>39°C) with rigors
Severe myalgia and arthralgia — "whole body ache"
Headache (often severe, frontal)
Dry cough, sore throat
Profound fatigue and malaise — may last weeks
Dyspnoea, pleuritic chest pain (suggests pneumonia)
Signs
Fever, flushed face
Pharyngitis
Signs of dehydration in severe cases
Crackles on auscultation (secondary bacterial pneumonia or viral pneumonitis)
Investigations
First-line
Clinical diagnosisDuring known influenza circulation, clinical diagnosis is appropriate for otherwise healthy individuals
Influenza PCR (nasopharyngeal swab)Confirm in: hospitalised patients, outbreaks, at-risk groups. Rapid PCR available in most hospitals. Also detects influenza type (A vs B) and subtype
Second-line
CXRIf secondary bacterial pneumonia suspected (persistent fever, productive cough, new consolidation)
FBC, U&Es, CRPIf hospitalised — assess for secondary infection and organ dysfunction
Specialist
Viral culture and sequencingFor surveillance and characterisation — not routine clinical practice
1
Self-care (most cases)
- Rest, fluids, paracetamol/ibuprofen for symptomatic relief
- Most healthy adults recover within 1-2 weeks
- Advise to stay home for 5 days from symptom onset to reduce transmission
2
Antiviral treatment (at-risk groups)
- Oseltamivir (Tamiflu) 75 mg BD for 5 days — most effective if started within 48 hours
- Indicated for: at-risk groups (pregnancy, chronic disease, immunosuppressed, elderly) AND when national surveillance confirms flu is circulating
- Can be started >48 hours in immunocompromised or hospitalised patients
- Zanamivir (inhaled) as alternative if oseltamivir not suitable
3
Prevention — annual vaccination
- Inactivated injectable vaccine for adults, live attenuated nasal spray (Fluenz) for children aged 2-17
- Target groups: ≥65 years, pregnant women, chronic disease (asthma, COPD, diabetes, CKD, heart disease), immunosuppressed, healthcare workers, carers
- Vaccination timing: September-November (before winter peak)
- Annual vaccination needed due to antigenic drift (minor mutations in surface proteins)
4
Secondary bacterial pneumonia
- Suspect if: fever initially improves then returns ("double sickening"), productive purulent cough, new consolidation on CXR
- Most common secondary organism: Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae
- Treat with appropriate antibiotics (e.g. co-amoxiclav or doxycycline)
Complications
- Secondary bacterial pneumonia: Most important complication — a leading cause of influenza-related death
- Viral pneumonitis: Primary influenza pneumonia — can cause ARDS
- Myocarditis: Rare but potentially fatal
- Exacerbation of chronic disease: Asthma, COPD, heart failure — major driver of winter hospital admissions
- Febrile convulsions: In children
- Encephalitis: Very rare
UKMLA Exam Tips
- 1Influenza vs common cold: flu = SUDDEN onset, HIGH fever, SEVERE myalgia, PROSTRATION. Cold = gradual onset, mild fever, sneezing, nasal symptoms predominate
- 2Oseltamivir within 48 hours of symptom onset — indicated for at-risk groups during flu season
- 3Antigenic DRIFT = minor mutations (seasonal variation). Antigenic SHIFT = major reassortment (pandemic potential — influenza A only)
- 4"Double sickening" pattern (initial improvement then deterioration) = suspect secondary bacterial pneumonia
- 5Live attenuated nasal vaccine (Fluenz) for children. Inactivated injectable vaccine for adults and immunocompromised
- 6S. aureus pneumonia post-influenza = rapid cavitating consolidation — high mortality
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