Immunizations - seasonal influenza
Exam Tips
- In UK exams, know who is offered NHS flu vaccine and the seasonal timing (offer from early autumn; ideally done by end of November).
- Key distinction: LAIV is intranasal live vaccine (mainly children), while most adult vaccines are inactivated IM products.
- Always mention high-risk groups explicitly (pregnancy, age >=65, chronic disease, immunosuppression, care-home residents, carers/close contacts of immunocompromised people).
- If asked about safety, state major LAIV contraindications and that vaccination is usually still possible in egg allergy via appropriate product selection.
- For severe influenza questions, add complications and escalation criteria rather than treating as a simple self-limiting viral URTI.
Definition
Seasonal influenza is an acute viral infection of the respiratory tract, predominantly caused by influenza A and B strains that circulate each winter in the UK. In most healthy people it is self-limiting over about 2-7 days, but in older adults, pregnancy, and defined clinical risk groups it can lead to severe complications, so annual vaccination is the key preventive intervention in UK primary care.
Pathophysiology
Influenza viruses infect respiratory epithelial cells via haemagglutinin-mediated entry, causing cell death, mucosal inflammation, and cytokine-driven systemic symptoms (fever, myalgia, malaise). Antigenic drift produces seasonal strain changes, which is why vaccine composition is updated annually (typically quadrivalent: 2 influenza A subtypes and 2 influenza B lineages). Severe disease is linked to impaired host reserve or immunity, with risk of viral pneumonitis, secondary bacterial pneumonia, encephalopathy/encephalitis, myocarditis, and decompensation of chronic disease. For revision, see the standard virion structure and antigenic drift/shift diagrams in core microbiology texts (figure showing HA/NA surface proteins).
Risk Factors
- Age 65 years and over
- Pregnancy (any trimester)
- Chronic respiratory disease (including steroid-treated asthma, COPD, bronchiectasis, cystic fibrosis, interstitial lung disease)
- Chronic heart or vascular disease (including heart failure, ischaemic heart disease, atrial fibrillation, peripheral vascular disease)
- Chronic kidney disease stage 3-5, nephrotic syndrome, renal transplant
- Chronic liver disease (for example cirrhosis, chronic hepatitis)
- Chronic neurological or neuromuscular disease affecting respiratory function
- Diabetes mellitus (type 1 or type 2) and adrenal insufficiency requiring steroid replacement
- Immunosuppression (disease- or treatment-related, including chemotherapy, biologics, post-transplant, HIV)
- Residence in long-stay care facilities
- Close household contact of severely immunocompromised person
- Children aged 2-3 years and school-aged children in national programme cohorts
Clinical Features
Symptoms
- Abrupt fever and chills
- Myalgia and headache
- Dry cough, sore throat, rhinorrhoea
- Marked fatigue and malaise
- Reduced appetite; sometimes nausea/vomiting (especially in children)
- Worsening baseline symptoms in chronic cardiorespiratory disease
Signs
- Pyrexia and tachycardia
- Injected pharynx, coryzal features
- Diffuse wheeze or crackles if lower respiratory involvement
- Hypoxia or increased work of breathing in severe disease
- Clinical dehydration in frail or older adults
Investigations
Management
Lifestyle Modifications
- Offer vaccination early each season (ideally completed by end of November, but can still vaccinate up to 31 March).
- Advise hand hygiene, respiratory etiquette, and staying away from vulnerable contacts while symptomatic.
- Use safety-netting: seek urgent review for breathlessness, chest pain, confusion, persistent fever, reduced urine output, or oxygen desaturation.
Pharmacological Treatment
Seasonal influenza vaccines (prevention)
- Live attenuated quadrivalent influenza vaccine intranasal (Fluenz Tetra) 0.2 mL intranasally as 0.1 mL per nostril, single annual dose (children eligible for LAIV)
- Inactivated quadrivalent influenza vaccine IM (egg-based or cell-based, brand dependent) usually 0.5 mL IM as a single annual dose from age 6 months
- Adjuvanted inactivated quadrivalent influenza vaccine (aQIV) 0.5 mL IM single annual dose (commonly used in older adults)
Select product by age and eligibility in the current NHS seasonal programme. Contraindications/warnings: defer in acute febrile illness; do not give LAIV in severe immunosuppression, salicylate therapy, or pregnancy; avoid LAIV with severe active wheeze/asthma exacerbation until stable; observe standard anaphylaxis precautions for all vaccines. Egg allergy: most people can receive influenza vaccine in primary care with an appropriate product pathway.
Antivirals for treatment/prophylaxis in selected high-risk scenarios
- Oseltamivir 75 mg orally twice daily for 5 days (treatment, adults)
- Oseltamivir 75 mg orally once daily for 10 days (post-exposure prophylaxis, adults)
- Zanamivir inhaled 10 mg (2 inhalations) twice daily for 5 days (treatment alternative)
Use according to UK national antiviral guidance, timing from symptom onset/exposure, and circulating strain susceptibility. Adjust oseltamivir in renal impairment. Zanamivir inhaler is generally unsuitable in severe underlying airway disease and in people unable to use inhaler devices effectively.
Complications
- Primary viral pneumonia
- Secondary bacterial pneumonia
- Exacerbation of asthma/COPD/heart failure
- Otitis media (children)
- Myocarditis
- Encephalitis/encephalopathy
- Meningitis (rare)
- Sepsis and death in vulnerable groups
Prognosis
Most healthy patients recover fully within about one week, though cough and fatigue can persist longer. Prognosis worsens with advanced age, frailty, pregnancy, multimorbidity, and immunosuppression; timely vaccination substantially reduces severe outcomes, hospitalization, and mortality at population level.
Sources & References
💊BNF Drug References(2)
- Influenza vaccine (inactivated)[management.pharmacological]
- Influenza vaccine (live)[management.pharmacological]
✅NICE Guidelines(1)
- Immunizations - seasonal influenza[overview]
📖Textbook References(6)
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 581)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 581)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1744)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1501)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 580, 581)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1500, 1501)[context]