Key Facts

Influenza is a major cause of morbidity and mortality in the UK, particularly during winter months. Vaccination remains the most effective preventive strategy. Antiviral treatment should be initiated promptly in at-risk patients.

Epidemiology

• UK: ~10,000–25,000 excess winter deaths attributable to influenza in severe seasons
• Global: 3–5 million severe cases/year; 290,000–650,000 deaths/year (WHO)
• Seasonal peaks: November–March in Northern Hemisphere
• Attack rate: 5–10% of adults, 20–30% of children annually
• Pandemics: H1N1 (2009), H3N2 (1968), H2N2 (1957), H1N1 (1918 — 50 million deaths)

Aetiology

• Influenza A: subtypes defined by haemagglutinin (H1–18) and neuraminidase (N1–11); infects humans, birds, pigs
• Influenza B: two lineages (Victoria, Yamagata); humans only; typically milder
• Influenza C: mild, sporadic
• Transmission: respiratory droplets, aerosols, fomites; incubation 1–4 days; infectious from 1 day before to 5–7 days after symptom onset

Pathophysiology

• Virus binds sialic acid residues on respiratory epithelial cells via haemagglutinin
• Neuraminidase cleaves sialic acid → release of progeny virions → spread
• Epithelial cell death → mucosal inflammation → susceptibility to secondary bacterial infection
• Systemic inflammatory response: cytokine release → fever, myalgia, malaise
• Severe disease: viral pneumonitis → ARDS; secondary bacterial pneumonia (S. aureus, S. pneumoniae)
• Antigenic drift: point mutations in HA/NA genes → seasonal epidemics (need annual vaccine update)
• Antigenic shift: reassortment between human and animal influenza A strains → novel virus → pandemic

Typical Presentation

• Sudden onset high fever (>38.5°C), rigors
• Severe myalgia and arthralgia
• Headache
• Dry cough
• Sore throat
• Prostration and malaise (often more severe than common cold)
• Duration: 5–7 days (cough may persist 2 weeks)

Complications

• Primary viral pneumonia: progressive dyspnoea, bilateral infiltrates → ARDS
• Secondary bacterial pneumonia: classically S. aureus (rapid onset, cavitation) or S. pneumoniae (after initial improvement)
• Myocarditis/pericarditis: chest pain, troponin elevation
• Encephalitis/encephalopathy: rare, confusion, seizures
• Rhabdomyolysis: myalgia, raised CK, AKI
• Reye syndrome: hepatic encephalopathy in children given aspirin
• Exacerbation of chronic disease: COPD, asthma, heart failure

Red Flags

• Dyspnoea, tachypnoea, SpO2 <92%
• Haemoptysis
• Persistent fever >5 days (secondary infection)
• Confusion or altered consciousness
• Dehydration, unable to tolerate oral fluids
• High-risk patient with typical influenza symptoms

Bedside

• Nasopharyngeal swab: influenza PCR (gold standard) — also tests for COVID-19 and RSV (multiplex PCR)
• Point-of-care rapid influenza test: less sensitive (~60–70%) but fast (15 min)
• SpO2: assess severity
• NEWS2: escalation tool

Bloods

• FBC: lymphopenia, relative leucopenia (high WCC suggests secondary bacterial infection)
• CRP/procalcitonin: CRP moderately raised; procalcitonin low in viral, high if bacterial superinfection
• U&Es, LFTs: assess organ function
• CK: if rhabdomyolysis suspected
• Blood cultures: if secondary bacterial infection suspected
• Blood gas: if respiratory compromise

Imaging

• CXR: if pneumonia suspected — bilateral interstitial infiltrates (viral) or lobar consolidation (bacterial)
• CT chest: if complicated pneumonia, ARDS

Special Tests

• Viral typing/subtyping: for surveillance (PHE/UKHSA)
• Sputum culture: if secondary bacterial pneumonia suspected

Non-pharmacological

• Rest and hydration: adequate oral fluid intake
• Isolation: respiratory droplet precautions; stay home for ≥5 days from symptom onset
• Infection control in hospital: surgical mask, hand hygiene, single room if possible

Pharmacological

Antivirals (NICE TA168):

• Oseltamivir (Tamiflu) 75mg BD for 5 days: indicated in at-risk patients within 48 hours of symptoms
• Zanamivir (Relenza) 10mg BD inhaled for 5 days: alternative if oseltamivir contraindicated
• Consider in hospitalised patients even if >48 hours from onset (may still reduce duration)
• Post-exposure prophylaxis: oseltamivir 75mg OD for 10 days for close contacts of confirmed cases who are at risk

Symptomatic treatment:

• Paracetamol/ibuprofen for fever and myalgia
• Avoid aspirin in children <16 (Reye syndrome risk)

Secondary bacterial pneumonia:

• Treat per NICE NG138 (community-acquired pneumonia guidelines)
• Co-amoxiclav 625mg TDS or doxycycline 200mg loading then 100mg OD
• If severe/S. aureus suspected: flucloxacillin 1g QDS IV ± gentamicin

Vaccination (annual):

• Quadrivalent inactivated vaccine: ≥65 (adjuvanted — aQIV), pregnant women, at-risk adults
• Live attenuated nasal spray (LAIV): children aged 2–10
• Healthcare workers: annual vaccination recommended
• Timing: September–November (before flu season peak)

Surgical/Interventional

• Not applicable for uncomplicated influenza
• Ventilatory support (NIV/invasive) for ARDS
• ECMO: in refractory ARDS (specialist centres)

Referral Criteria

• Hospital admission: severe symptoms, SpO2 <92%, dehydration, complications
• ICU: ARDS, septic shock, multi-organ failure
• Public health notification: not individually notifiable but outbreaks are reportable

• Uncomplicated influenza: self-limiting in 5–7 days in most healthy adults
• Hospitalisation rate: ~1% overall, much higher in elderly and high-risk groups
• UK excess winter deaths attributable to influenza: 10,000–25,000 in severe seasons
• Secondary bacterial pneumonia mortality: 10–30%
• ARDS from influenza: ICU mortality ~30–40%
• Vaccination reduces hospitalisation by 40–60% (higher in well-matched seasons)
• Oseltamivir: reduces symptom duration by ~1 day and reduces complications in at-risk groups
• Pandemic influenza (e.g. 1918 H1N1): case-fatality rate was ~2–3% (50 million deaths globally)
• Children: generally good prognosis; rare fulminant cases with encephalopathy

UK Influenza Vaccination Programme

Influenza vs Common Cold