Influenza

Overview

•Highly contagious acute respiratory illness caused by influenza A or B viruses

•Influenza A - responsible for most seasonal epidemics and all recorded pandemics

•Incubation period 1-4 days (typically 2 days); contagious from ~1 day before symptoms until 5-7 days after onset

Presentation

•Abrupt onset - hallmark feature (patient well in morning, floored by afternoon)

•Fever - sudden onset, 38-40°C, almost universal

•Myalgia - severe, particularly legs and back; defining feature

•Headache - prominent and early

•Dry cough - persistent

•Sore throat and coryza - present but less prominent than common cold

•Fatigue and malaise - can be severe and prolonged (weeks)

•Febrile convulsions - in young children with rapid temperature rise

Influenza vs common cold

Feature	Influenza	Common cold
Onset	Sudden (hours)	Gradual
Fever	High (38-40°C), almost universal	Low-grade or absent
Myalgia	Severe	Mild or absent
Nasal symptoms	Minor	Prominent
Systemic illness	Prominent	Absent/mild

Investigations

•Clinical diagnosis - sufficient in community settings during seasonal circulation; no testing needed to start antivirals in at-risk patients

•Rapid influenza diagnostic test (RIDT) / point-of-care PCR - used in hospitals and care homes for infection control

🏆 Gold standard

•RT-PCR of nasopharyngeal swab - highest sensitivity and specificity; differentiates A from B; identifies subtype; used for public health surveillance

•Chest X-ray - if pneumonia suspected (consolidation; bilateral interstitial infiltrates in viral pneumonia)

•FBC, U&E, CRP, LFTs - in hospitalised patients to assess severity

Management

•Healthy adults: supportive care - rest, hydration, paracetamol or ibuprofen for fever/myalgia; full recovery in 1-2 weeks

•At-risk patients: oseltamivir 75 mg twice daily for 5 days - start within 48 hours of symptom onset without waiting for confirmation

•Zanamivir (inhaled) - alternative if oseltamivir not tolerated or resistance suspected

🚨

Do NOT give aspirin to children under 16 with influenza - associated with Reye syndrome (rare but potentially fatal hepatic encephalopathy). Use paracetamol or ibuprofen instead.

Prevention

•Annual influenza vaccination - vaccine reformulated yearly due to antigenic drift; typically includes 2 influenza A subtypes and 1-2 influenza B strains

•Eligible groups: all adults ≥65; all pregnant women; chronic respiratory, cardiac, renal, hepatic, neurological, or metabolic conditions; immunosuppressed; BMI ≥40; residents of long-stay care facilities; carers; all healthcare workers

•Children (2-17 years): live attenuated intranasal influenza vaccine (LAIV) as part of routine childhood immunisation

•Inactivated injectable vaccine instead in children with severe asthma or who are immunosuppressed

•Adults ≥65: adjuvanted quadrivalent inactivated vaccine (QIVe) - produces stronger immune response given age-related immunosenescence

Complications

•Primary viral pneumonia - bilateral interstitial infiltrates; can progress to ARDS; higher mortality than secondary bacterial pneumonia

•Secondary bacterial pneumonia - occurs 5-10 days after initial improvement; caused by Streptococcus pneumoniae, Staphylococcus aureus (including MRSA), or Haemophilus influenzae; presents with recurrence of fever and productive cough

•Exacerbation of chronic conditions - COPD, asthma, heart failure, ischaemic heart disease

•Reye syndrome - rare hepatic encephalopathy in children; associated with aspirin use during influenza

•Influenza encephalitis/encephalopathy, myocarditis, pericarditis - rare

💡

Secondary bacterial pneumonia classically presents as a biphasic illness - initial improvement followed by recurrence of fever with productive cough 5-10 days into the illness.

Virology - Key Concepts

•Haemagglutinin (HA) - mediates viral attachment to sialic acid receptors on respiratory epithelium

•Neuraminidase (NA) - cleaves receptors to allow release of new viral particles

•Antigenic drift - gradual point mutations in HA/NA; partially evades existing immunity; reason vaccine must be reformulated annually

•Antigenic shift - reassortment of whole gene segments between two influenza A strains; produces novel subtype with no population immunity; mechanism behind pandemic influenza