17th February 2025

Influenza

Influenza is the commonest cause of viral pneumonia in the immunocompetent and is especially prevalent during the flu season surrounding winter. In the UK, vaccination programmes aim to predict the likely strains that will have the most clinical impact in a given year. The influenza A virus is responsible for the majority of hospital illnesses; the H1N1 subtype has been involved in numerous epidemics including the Spanish flu and the 2009 swine flu outbreak. 

Diagnosis tends to be based on fever, myalgia, tachypnoea and wheeze. Often, superadded bacterial infections complicate the clinical picture. Diagnosis may be achieved through PCR or culture taken from swabs. Imaging often shows bilateral interstitial infiltrates unless complicated by a bacterial infection with focal consolidation. 

Treatment in the first instance is supportive, although intubation may be required in extremis. In the immunosuppressed or otherwise vulnerable presenting within the first 48 hours of symptoms, oseltamivir or other neuraminidase inhibitors reduce illness duration.