Clinical features

For healthy individuals the infection is an unpleasant but usually self-limiting disease and patients usually recover within 2–7 days.

• Serological studies have revealed that around 30 to 50% of influenza infections can be asymptomatic, however, this may vary depending on the characteristics of the influenza strain (1).

In mild, uncomplicated influenza, after an incubation period of between one and four days, there is an abrupt onset of:

• fever – usually it ranges between 38–40°C, the peak occurs within 24 hours of onset and lasts for 3 days
• malaise
• headache
• chills
• cough – generally dry but may be productive in some cases
• myalgia – mainly the back and the limbs
• extreme fatigue
• nasal congestion
• sore throat
• anorexia
• gastrointestinal symptoms – such as vomiting and diarrhoea are uncommon (10%) in adults (1,2)

Symptoms may vary in infants and children and may include fatigue, irritability, diarrhoea and vomiting (3).

After three to seven days the fever ceases and illness typically resolves although cough, malaise and lassitude may persist for a longer period (2).

Although the infection is usually self limiting it may cause complications such as primary influenza viral pneumonia, exacerbation of underlying medical conditions (e.g., pulmonary or cardiac diseases) and co-infections with other viral or bacterial pathogens (4).

Reference:

1. Immunisation Against Infectious Disease - "The Green Book". Chapter 19. Influenza (October 2020)
2. British Infection Society; British Thoracic Society; Health Protection Agency. Pandemic flu: clinical management of patients with an influenza-like illness during an influenza pandemic. Provisional guidelines from the British Infection Society, British Thoracic Society, and Health Protection Agency in collaboration with the Department of Health. Thorax. 2007;62 Suppl 1:1-46.
3. National Institute for Health and Clinical Excellence (NICE) 2009. Amantadine, oseltamivir and zanamivir for the treatment of influenza (review of NICE technology appraisal guidance 58)
4. Centers for Disease Control and Prevention (CDC). 2009-10 Influenza Prevention & Control Recommendations Clinical Signs and Symptoms of Influenza