Influenza (seasonal) for health professionals

Influenza, commonly known as the flu, is a highly contagious infection of the nose, throat and lungs caused by the influenza A or B (or rarely C) viruses.

This page covers seasonal influenza and not pandemic influenza or highly pathogenic avian influenza.

Clinical presentation

In South Australia, influenza is most common in the winter months.

Influenza causes a spectrum of illness from asymptomatic infection to fatal pneumonia (primary viral or secondary bacterial). Severe disease is more common in pregnancy, people aged over 65 years and in people with underlying chronic disease.

Doctors should consider influenza in anyone with:

  • Fever (greater than 38oC or a good history of fever)
  • Cough or sore throat, in the absence of any other explanation for symptoms

However, fever may be absent in elderly persons.

Most people with uncomplicated influenza recover within a week, although the cough and fatigue may last longer.

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Mode of transmission

Spread is via respiratory droplets and direct contact with respiratory secretions.

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Incubation period

Incubation period is 1 to 4 days (average 2 days).

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Infectious period

People infected with influenza are considered infectious from 1 day before onset of symptoms, and viral shedding is greatest in the first 3 to 5 days of illness. Young children shed virus for longer, up to 7 to 10 days; shedding may continue for longer in severely immunocompromised persons.

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In mild illness laboratory testing is not always necessary.

Take a deep nasal or throat swab, or sputum for respiratory viral testing. Use appropriate personal protective equipment when obtaining the sample.

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  • Encourage annual influenza vaccination. Vaccination against influenza has the primary aim of reducing severe influenza. The influenza vaccine is recommended for any person 6 months of age and over who wishes to reduce the likelihood of becoming ill with seasonal influenza. Due to changes in circulating strains and waning immunity, to minimise the chance of developing influenza, vaccination is required each year.
  • Give influenza vaccine to pregnant women - it is safe in all trimesters.
  • Exclude cases from childcare, preschool, school or work until there has been no fever for 24 hours (without using an antipyretic such as paracetamol).
  • Encourage respiratory etiquette and hand hygiene:
    • cover a cough or sneeze with a tissue or arm (not a hand)
    • drop used tissues immediately into a rubbish bin
    • wash hands with soap and running water, or use an alcohol based hand rub, after sneezing, coughing or touching used tissues.
  • In healthcare settings, institute contact and droplet precautions.
  • Consider prescribing antivirals. Antivirals can be given after known exposure to influenza. Prophylactic antivirals are usually only used in persons at high risk of severe influenza. If used, prophylaxis should be commenced within 48 hours of exposure.

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  • Medically assess the need for hospital presentation/admission.
  • consider antivirals for treatment in:
    • patients with established complications or severe illness (for instance, needing hospitalisation)
    • patients from high risk groups
    • patients not in high risk groups and not severely unwell if commenced within 48 hours of onset.
  • patients with uncomplicated influenza do not require antibiotics.

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The Communicable Disease Control Branch, South Australia should be notified within 3 days of suspicion or confirmation of influenza on either on:

  • telephone: 1300 232 272
  • fax: (08) 8226 7187

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The Communicable Disease Control Branch conducts state-wide surveillance of all notifiable conditions (including influenza) in South Australia. This includes a chart that provides a regular update on influenza activity in South Australia.

Nationally, the Australian Influenza Surveillance Report and Activity Updates provides national data monitoring influenza activity and severity.

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Resources and useful links

Need fact sheets, posters and clinical tools? Take advantage of our list of influenza resources and links to other government websites.

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