🧭 When to suspect

Suspect influenza in any patient presenting during the influenza season (typically late autumn to early spring) with the abrupt onset of fever, dry cough, headache, myalgia and marked malaise. When national surveillance confirms that influenza is circulating above baseline, influenza-like illness (ILI) remains a clinical diagnosis in primary care, and routine swabbing is not required. The pretest probability is high during the season, but co-circulating respiratory viruses (notably COVID-19 and respiratory syncytial virus) can be clinically indistinguishable, so the diagnosis still rests on the epidemiological picture.

A history of influenza vaccination does not exclude the diagnosis. The two essential primary-care tasks are to identify patients in a clinical risk group, who may benefit from prompt antiviral treatment, and to never miss the complications – pneumonia, sepsis and meningoencephalitis. Smoking is an important modifiable risk factor for severe disease.

Clinical risk group (eligible for antivirals)	Includes
Chronic respiratory disease	Asthma needing continuous or repeated inhaled/systemic steroids or prior hospital admission; chronic obstructive pulmonary disease (COPD); bronchiectasis; cystic fibrosis
Chronic heart disease	Heart failure, ischaemic heart disease, congenital heart disease
Chronic kidney, liver or neurological disease	CKD stage 3–5; cirrhosis or chronic hepatitis; stroke or transient ischaemic attack, Parkinson's, motor neurone disease, multiple sclerosis, learning disability
Diabetes mellitus	Type 1 and type 2
Immunosuppression	Chemotherapy or radiotherapy, biologics, high-dose systemic steroids, HIV, solid-organ or bone-marrow transplant
Asplenia or splenic dysfunction	Splenectomy, sickle cell disease
Other higher-risk groups	Morbid obesity (body mass index, BMI ≥ 40); pregnancy (including up to 2 weeks post-partum); age ≥ 65; children under 6 months

Source: NICE TA168 · UKHSA · Influenza Green Book chapter 19

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