Influenza World Health Care Professionals

Timecourse of influenza

The incubation period for influenza is usually 2–3 days, but in some cases it may be up to 1 week. [1] A sudden onset of severe symptoms (within 3–6 hours) correlates with the peak of viral replication, 1–3 days after infection. [1]

Systemic symptoms (malaise, feverishness, chills, headache, anorexia, myalgia, and dizziness) typically last for 3–4 days. [1] These early symptoms are usually accompanied by respiratory symptoms (non-productive cough, nasal discharge, sneezing and a sore throat). Cough and malaise may persist for 2 weeks after fever dissipates. [1]
Adults with influenza may continue to shed virus for up to 6 days, whereas in children shedding may be prolonged to 7–10 days. [2] During this time they remain infectious to others.
A graphical timeline of influenza – from infection through to disease symptoms and improvement is shown below. 

Influenza symptoms

Influenza is common in children, though it can be difficult to recognise influenza in young children as they can find it hard to describe their symptoms. [3] Also, influenza can be easily confused with other childhood infections (e.g. respiratory syncytial virus bronchiolitis). [4]
The common symptoms of influenza in adults and young children are shown in the table below. [5]

	Adults	Childern may also experience
Symptoms	Fever	Nausea
	Headache	Vomiting
	Extreme fatigue	Diarrhoea
	Myalgia
	Dry cough
	Sore throat (less common)
	Rhinitis (less common)
Complications	Bacterial pneumonia	Bacterial pneumonia
	Bronchitis	Otitis media
	Sinusitis	Worsening of chronic medical conditions
	Dehydration	Febrile convulsions

 

Unfortunately, many people believe influenza cannot be treated. However, it is important for patients to be able to recognise the symptoms of influenza and understand that by seeking medical attention immediately, antiviral medications available on prescription can be used to effectively treat and prevent influenza.

The Flu FACTS are a useful tool to help patients remember the common symptoms of influenza.

• Fever
• Aches
• Cough
• Tiredness
• Sudden symptoms.

Influenza or the common cold?

Influenza can easily be mistaken for a severe cold. However, the cold virus – rhinovirus – is less likely to cause the symptoms of influenza described above, and more likely to cause other symptoms such as sneezing and a sore throat. [6] These symptoms come on gradually in contrast to the rapid onset of symptoms seen with influenza.
The common symptoms of influenza versus the common cold are shown in the table below.[6]

SYMPTOM	COLD	INFLUENZA
Fever	Rare	Usually present
Myalgia	Slight	Usual, often severe
Coughing	Absent or may develop hacking, productive cough	Dry, unproductive cough
Extreme fatigue	Mild	Moderate-to-severe
Chills	Uncommon	Fairly common
Sneezing	Common	Uncommon
Rhinitis	Common	Uncommon
Sore throat	Common	May be present
Chest discomfort	Mild-to-moderate	Often severe
Headache	Uncommon	Common
Sudden symptoms	Appear gradually	Can appear suddenly (within 3–6 hours)

 

A short film on the signs and symptoms of influenza can be viewed here.

 

1. Nicholson K. Human influenza. In: Nicholson K, Webster RG, Hay A, eds. Textbook of influenza. Oxford, UK: Blackwell Science, 1998: 219–64.
2. Sato M, Hosoya M, Kato K, Suzuki H. Viral shedding in children with influenza virus infections treated with neuraminidase inhibitors. Pediatr Infect Dis J 2005; 24: 931–2.
3. Peltola V, Reunanen T, Ziegler T, Silvennoinen H, Heikkinen T. Accuracy of clinical diagnosis of influenza in outpatient children. Clin Infect Dis 2005; 41: 1198–200.
4. Fleming DM. Influenza diagnosis and treatment: a view from clinical practice. Philos Trans R Soc Lond B Biol Sci 2001; 356: 1933–43.
5. Centers for Disease Control and Prevention: Key facts about influenza and influenza vaccine, 2006. Available here.
6. US Dept. of Health and Human Services Centers for Disease Control and Prevention: Is it a cold or the flu? Available here