AKT · Infections/Viral infectionslow yield

Influenza

Influenza A/B virus — seasonal respiratory infection; antigenic drift/shift

Overview

An acute seasonal respiratory infection caused by influenza A or B, spread by droplets. Most cases are self-limiting, but it causes significant morbidity in at-risk groups (elderly, pregnant, chronic disease, immunosuppressed) — primarily through complications, especially secondary bacterial pneumonia (classically Staph. aureus). Management is supportive for the well; neuraminidase inhibitors (oseltamivir) are for at-risk patients within 48 hours, and annual vaccination is the key prevention.

Recognise

Abrupt fever, myalgia, headache, dry cough, sore throat, prostration (more systemic than a common cold); usually self-limiting in ~1 week
Complications: secondary bacterial pneumonia (Staph. aureus — cavitating, post-influenza), exacerbation of chronic disease, otitis media, rarely myocarditis/encephalitis
At-risk groups: ≥65, pregnant, chronic heart/lung/liver/renal/neuro disease, diabetes, immunosuppression, BMI ≥40

Red flags

Deterioration with new consolidation → secondary bacterial pneumonia (esp. Staph. aureus) → antibiotics
Severe influenza in at-risk patients → hospital care; pandemic strains → public-health measures

Differentials & how to tell them apart

Common cold / other viral URTImilder, less systemic, gradual onset

COVID-19overlapping — anosmia more COVID; PCR distinguishes

Bacterial pneumoniafocal consolidation — may be a secondary complication of influenza

Investigations

Usually clinical during flu season; PCR (nose/throat swab) confirms and is used in hospital/at-risk/outbreak settings; CXR if pneumonia suspected; bloods if severe.

Management

Supportive if well; oseltamivir within 48 h for at-risk/severe; annual vaccination to prevent

1Diagnose clinically in flu season (PCR if at-risk/hospitalised/outbreak). Otherwise-healthy patients → supportive care only (no antibiotics).Gate: At-risk groups or severe/hospitalised disease → oseltamivir within 48 hours; new consolidation/deterioration → secondary bacterial pneumonia (cover Staph. aureus).
2Treat complications (secondary pneumonia, exacerbations); prevent with annual vaccination of at-risk groups and infection-control measures.

Supportive care for otherwise healthy patients — rest, fluids, antipyretics — most are self-limiting; antibiotics NOT indicated for uncomplicated influenza

Oseltamivir (neuraminidase inhibitor) within 48 h for at-risk/severe — reduces duration/complications if started early in at-risk groups or hospitalised patients; zanamivir alternative

Antibiotics for secondary bacterial pneumonia — cover Staph. aureus in post-influenza cavitating pneumonia

Annual vaccination (prevention) + infection control — offered to at-risk groups, healthcare workers, children; the key preventive measure

Key points

Abrupt fever + myalgia + dry cough (more systemic than a cold) = influenza → supportive if well; oseltamivir within 48 h for at-risk/severe. The dangerous complication is secondary bacterial pneumonia (classically Staph. aureus, cavitating, post-flu). Annual vaccination is the key prevention. No antibiotics for uncomplicated flu.

Monitor & prognosis

Recovery/complications, at-risk deterioration; vaccination uptake.

Self-limiting in the healthy; complications drive morbidity/mortality in at-risk groups.

Source: NICE CKS / UKHSA influenza; cross-ref respiratory (pneumonia)