Uncomplicated influenza presents as follows: presence of fever, coryza, generalised symptoms (headache, malaise, myalgia, arthralgia) and sometimes gastrointestinal symptoms, but without any features of complicated influenza.

Complicated influenza can be distinguished as follows:

• Difficulty in breathing or chest pain
• Signs of lower respiratory tract infection (hypoxaemia, dyspnoea, lung infiltrate)
• Purple or blue discoloration of the lips
• Vomiting and inability to keep liquids down
• Signs of dehydration, such as feeling dizzy when standing and inability to urinate
• Seizures (uncontrolled convulsions)
• Confusion
• Changes in responsiveness

It is recommended that emergency medical care is sought if any of the above symptoms occurs in an elderly person or child with flu.

An influenza-like illness with an acute respiratory infection presents with the following parameters:

• a measured body temperature of ≥38°C
• cough with onset within the last 10 days

Author: Dr Reshma Ramracheya is a group leader at the University of Oxford with over 18 years of experience in teaching and medical research.

Reference:

• Hay AJ, McCauley JW. The WHO global influenza surveillance and response system (GISRS)—A future perspective. Influenza Other Respi Viruses. 2018;12:551–557. https://doi.org/10.1111/irv.12565.
• Hemila H., Chalker E., Douglas B. The Cochrane Library; 2010.
• Sabbaghi A, Miri SM, Keshavarz M, Zargar M, Ghaemi A. Inactivation methods for whole influenza vaccine production. Rev Med Virol. 2019 Jul 23:e2074. doi: 10.1002/rmv.2074.
• CDC, Centers for Disease Control and Prevention (https://www.cdc.gov/vaccines/pubs/pinkbook/flu.html)
• NICE, National Institute for Health and Care (https://www.nice.org.uk/guidance/conditions-and-diseases/infections/influenza)