Seasonal influenza for health professionals

Seasonal influenza is mainly caused by influenza A or B viruses. It is a nationally notifiable respiratory disease. We monitor and report on national case numbers and epidemiological trends. We publish case definitions and national guidelines to support health professionals and health authorities.

For health professionals For everyone

Disease

  • There are 4 types of influenza viruses: A, B, C and D.

    Influenza virus types A, B and C can infect humans, but A and B viruses are the main cause of seasonal influenza. 

    Only influenza A viruses have caused influenza pandemics.

    Influenza A has subtypes, based on 2 proteins on the surface of the virus: 

    • hemagglutinin (H) – 18 subtypes
    • neuraminidase (N) – 11 subtypes. 

    More than 130 influenza A subtype combinations have been identified, but there are likely many others. The most common subtypes currently circulating in people are H1N1 and H3N2.

    Influenza A viruses are constantly changing through: 

    • antigenic drift – small genetic changes that occur continuously as influenza viruses replicate
    • antigenic shift – abrupt, major genetic changes resulting in new H or new H and N proteins in viruses; these result from an animal influenza virus becoming able to infect humans.

    Specific types of influenza A can also cause avian influenza in humans.

    Because of the rapid evolution of influenza viruses, vaccines need to be reviewed each year to combat the most common strains.

    The term ‘seasonal influenza’ refers to influenza viruses that circulate every year, mainly in winter. This is different to pandemic influenza, which happens when a new influenza virus strain emerges, usually as a result of antigenic shift. Because people have little or no immunity to the new strain, transmission rates are very high, causing widespread illness.

    Read more about:

  • Influenza illness ranges from asymptomatic infection to severe disease. 

    Clinical features typically include:

    • fever (usually 38°C or higher)
    • rhinorrhoea
    • chills
    • cough
    • headache
    • myalgia
    • sore throat 
    • fatigue.

    Diarrhoea and vomiting may also occur, more commonly in children.

    Elderly people might present with no fever and with atypical symptoms, such as anorexia or mental status changes.

    Serious complications can include:

    Read more about symptoms, diagnosis and treatment of influenza.

Public health importance

  • On average, each year in Australia, seasonal influenza results in an estimated:

    • 3,500 deaths
    • 300,000 general practitioner consultations 
    • 18,000 hospitalisations. 

    But these are underestimates of the true impact.

    Rates of influenza hospitalisation and mortality are significantly higher for certain priority groups. 

    Surveillance and monitoring for influenza is important, because new strains can evolve with the potential to cause a pandemic. 

    Influenza pandemics are rare – only 4 have occurred in the past 100 years. 

  • Apart from the years 2020 and 2021 – when influenza cases reduced during the COVID-19 pandemic – Australia experiences high numbers of seasonal influenza cases annually. 

    Influenza infections are generally most common between June and September, peaking around August.

    The number of cases and disease severity varies from year to year, depending on:

    • the circulating virus 
    • the population’s susceptibility 
      • influenza A H3N2 affects older people more heavily
      • influenza A H1N1 and influenza B affect children, pregnant women and younger adults more heavily
    • the level of immunity of the population – from vaccination and past infection
    • the effectiveness of the vaccine.

    For the latest information on seasonal influenza incidence, severity, transmission and virology in Australia see the:

Spread of infection

  • Person-to-person transmission most commonly occurs by breathing in respiratory droplets containing the influenza virus. 

    Aerosol transmission can occur in aerosol-generating procedures and enclosed spaces, but it is less common.

    Indirect transmission – such as from fomites – can also occur. Influenza viruses can persist on surfaces for several hours.

  • The incubation period ranges from 1 to 4 days but about 2 days is most common.

  • The infectious period can start up to 24 hours before the onset of symptoms until 7 days after symptom onset. 

    The infectious period varies between populations:

    • Adults are most infectious in the first 2 days after the onset of symptoms. This reduces to low levels by 5 days after symptom onset.
    • Children can be infectious for 10 or more days.
    • People who are immunocompromised can be infectious for weeks. They should maintain infection prevention and control precautions for longer.

    Adult influenza patients are no longer considered infectious 24 hours after their fever has ended without anti-pyretic medication, and it has been either:

    • 72 hours since they started antiviral medication 
    • 5 days since the onset of respiratory symptoms.

Priority populations

Prevention

  • Annual vaccination is recommended for:

    Commercial poultry and pork industry workers are also recommended to have an influenza vaccination during an outbreak of avian or swine flu. This is to prevent mixing of seasonal influenza viruses with avian or swine strains if a person is infected with both at the same time.

    See the Immunisation Handbook for more information, and who is eligible for free influenza vaccination under the National Immunisation Program.

  • Other prevention measures include good hygiene practices, including: 

    • hand hygiene
    • respiratory hygiene
    • regular cleaning and disinfection of frequently touched objects and surfaces.

    People with acute respiratory symptoms should stay home and avoid:

    • public gatherings
    • crowded settings
    • public transport
    • school, childcare or work
    • residential care facilities or hospitals.

    Evidence-based recommendations on infection prevention and control in hospitals and other healthcare settings is provided by the Australian Commission on Safety and Quality in Health Care. See the Australian Guidelines for the Prevention and Control of Infection in Healthcare. 

  • It is particularly important for people at increased risk of severe illness to:

    In the lead up to winter, extra public health initiatives may be implemented aimed at preventing influenza infections, with a focus on these groups. This may include:

    • encouraging people to get vaccinated
    • educating people on how to prevent the spread of infection
    • encouraging people to see their healthcare provider to plan for testing and access to antiviral medications.

Diagnosis and clinical management

  • Influenza is usually diagnosed through laboratory testing. Polymerase chain reaction (PCR) is the most common test in Australia. This test involves taking samples from the nose and throat. If there are conjunctival (eye) symptoms, a conjunctival swab may sometimes be collected.

    Rapid antigen tests (RATs) that detect influenza are also widely available. These tests are not as sensitive as PCR at detecting infections. 

    Influenza can also be diagnosed using serological testing (via a blood test), although this is more often done for surveillance purposes than routine diagnosis. Samples taken for serology should be taken during the first 7 days from symptom onset.

  • Read more about laboratory testing and the laboratory case definition.

  • Influenza infections are normally self-limiting, and most people will recover in 1 to 2 weeks.

    Antiviral medications may be offered to people who develop severe influenza, including people who need to be hospitalised. Antivirals may also be considered in people at higher risk of poor outcomes from influenza. 

    Read more about the clinical management of influenza cases. 

Notification and reporting

Public health response

  • The CDNA National guidelines for public health units inform the public health response to seasonal influenza.

  • The public health management of cases focuses on preventing transmission. Public health messaging recommends that people with influenza should:

    • stay at home until acute symptoms have resolved
    • avoid contact with people at increased risk of severe disease
    • maintain appropriate hand and respiratory hygiene
    • take extra steps to minimise the risk of transmission to others if leaving home is necessary, such as
      • wearing face masks
      • avoiding public transport and crowded, indoor areas.

    Specific settings – such as childcare, school, work and high-risk settings – may also have exclusion periods for influenza cases. In general, children should not go to school or childcare and adults should not go to work until 24 hours has passed with no fever (without fever-reducing medication).

    Public health units don’t normally follow up single notifications, but they might do so in some situations. This might be for a case with a novel influenza subtype or an infection that can’t be typed using normal laboratory methods.

    See the CDNA National guidelines for public health units for more information.

    Different guidelines apply to the management of avian influenza in humans or pandemic influenza.

  • Contact tracing and management is only required for seasonal influenza in some high-risk settings or special situations. 

  • How public health agencies respond to seasonal influenza cases and outbreaks depends on: 

    • state or territory legislation
    • local reporting requirements 
    • the nature of the cases or outbreak 
    • available resources. 

    The public health response to influenza outbreaks generally focuses on high-risk settings. 

    See the CDNA National guidelines for public health units for more information on the response to influenza outbreaks. 

    Control measures used during outbreaks may include:

    • encouraging vaccination
    • encouraging early identification of symptoms and immediate testing
    • providing antiviral prophylaxis, as appropriate
    • tailoring health education and messaging
    • isolating or grouping people with influenza symptoms
    • increasing infection control, such as
      • providing hand hygiene or personal protective equipment supplies
      • increasing surface cleaning 
      • laundering or throwing out soiled articles.

  • Special situations and high-risk settings might require extra infection prevention and control precautions. These include:

    See the CDNA National guidelines for public health units for detailed guidance.

Resources

We are adding new content to this website and expanding these pages, including a suite of disease-related information. In the meantime, information about diseases in Australia is available on the Department of Health and Aged Care’s website. 

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