Influenza

Influenza is an acute respiratory illness caused by influenza virus, with abrupt onset of systemic and respiratory symptoms. It occurs in seasonal epidemics and, less frequently, in pandemics arising from antigenic shift.

It carries significant morbidity and mortality, particularly in high risk populations. Unlike most respiratory viral illnesses, specific antiviral treatment and vaccination are available and clinically relevant.

Virology

Influenza A and B cause the great majority of human seasonal disease. Influenza A is further classified by its surface haemagglutinin and neuraminidase proteins.

Both antigenic drift and antigenic shift occur. Drift is gradual mutation and necessitates annual vaccine reformulation. Shift is major reassortment and carries pandemic potential, given the resulting lack of pre-existing population immunity.

The typical presentation is abrupt onset of fever, chills, myalgia, headache and profound fatigue, together with respiratory symptoms (cough, sore throat, nasal congestion).

Sudden onset and prominent systemic symptoms (particularly myalgia and fatigue) help distinguish influenza from the more gradual onset of most other upper respiratory viral illnesses. Clinical distinction alone is imperfect, and confirmatory testing is often required, particularly where the result would influence management.

Symptoms typically resolve within five to seven days in uncomplicated disease, though fatigue and cough can persist longer.

High risk groups for severe disease and complications include the elderly, young children, pregnant women, and those with chronic cardiac, respiratory, renal or immunocompromising conditions. Use a lower threshold for antiviral treatment and closer monitoring in these groups.

Complications include:

• Secondary bacterial pneumonia, a recognised and clinically important complication, particularly with Staphylococcus aureus and Streptococcus pneumoniae.
• Primary viral pneumonia.
• Exacerbation of underlying chronic cardiac or respiratory disease.
• Myocarditis and encephalitis, less commonly.

Rapid antigen testing or polymerase chain reaction testing on a nasopharyngeal swab confirms the diagnosis. Polymerase chain reaction has substantially better sensitivity. Confirmation matters most where it would influence antiviral treatment, infection control measures or further investigation for a high risk or hospitalised patient.

During known community influenza circulation, a patient with a characteristic presentation may reasonably be diagnosed clinically without testing in low risk, community managed presentations. Reserve confirmatory testing for higher risk patients, those requiring hospital admission, or where the result would specifically guide management.

Perform chest radiography and further investigation for secondary bacterial pneumonia or other complications where clinically indicated by the presentation, rather than routinely in uncomplicated influenza.

Supportive care is the foundation of management for the majority of presentations: adequate hydration, rest, and analgesia or antipyretics for symptomatic relief. Uncomplicated disease in low risk individuals is generally self limiting.

Antiviral treatment (neuraminidase inhibitors, such as oral oseltamivir or inhaled zanamivir) is most effective when started within forty eight hours of symptom onset. It is recommended for patients with severe disease, those requiring hospital admission, and those in high risk groups for complications.

Treatment outside this window is still considered for hospitalised or severely unwell patients, given ongoing viral replication in more severe disease.

Antibiotics have no role in the treatment of uncomplicated viral influenza. Reserve them for confirmed or strongly suspected secondary bacterial infection, most commonly bacterial pneumonia, guided by the clinical features suggesting this complication.

Annual vaccination remains the most effective preventive measure. It is recommended for high risk groups and, in many jurisdictions, offered more broadly to the general population. Vaccine composition is updated annually to reflect anticipated circulating strains based on global surveillance.

Infection control measures (respiratory hygiene, isolation of hospitalised patients with suspected or confirmed influenza) are important given the significant transmissibility of the virus, particularly in healthcare and other congregate settings.

Post exposure antiviral prophylaxis may be considered for high risk contacts following significant exposure, particularly within closed settings such as care homes, guided by local public health advice and the circumstances of the exposure.