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At a glance

About seasonal flu

Seasonal flu is a common and highly contagious respiratory infection. 

It is usually caused by influenza A or B viruses, but there are many subtypes and strains.

Seasonal flu can cause mild to severe illness. Most people recover within a few days, but some people are at greater risk of getting severely unwell from it.

Seasonal flu is caused by a different virus to COVID-19.

Seasonal flu vaccines are the best protection against severe illness from flu.

Why it matters to public health

Seasonal flu is a common cause of hospitalisation. Before the COVID-19 pandemic, it was the most common cause of notifiable disease deaths. 

Flu infections can vary widely from year to year. During the COVID-19 pandemic, flu cases were lower than usual, as COVID-19 restrictions reduced the spread of flu.

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

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

This has a major impact on:

  • individuals – through illness, missed school or work, complications like pneumonia, hospitalisation and death
  • our health system – through added demand on health professionals, clinics and hospitals
  • businesses and our economy – through lost work time and productivity.

Because of these major impacts, and the fact that new flu viruses can cause pandemics, we:

Symptoms

If you’re looking for advice about your own health or treatment options, see healthdirect or speak with a qualified healthcare professional. Our role is to provide public health advice – information and guidance that helps prevent disease, protect communities and improve wellbeing at a population level.

Common symptoms include:

Symptoms might range from mild to severe, and you might not get all of those symptoms.

healthdirect’s symptom checker can help you:

  • work out whether you have the flu or another illness
  • decide whether to see a health professional.

Check your symptoms

Onset of symptoms

Symptoms will usually show about 2 days after getting infected, but this can range from 1 to 4 days.

How it spreads

Seasonal flu spreads easily through droplets from your respiratory tract. When you do things like talk, cough or sneeze, these droplets disperse through the air and may land on people and surfaces.

Other people can catch seasonal flu by:

  • their mouth, nose or eyes coming into contact with these droplets
  • breathing in these droplets
  • touching surfaces or objects that have been in contact with an infected person, then touching their mouth, nose or eyes. 

Read more about how seasonal flu spreads.

Infectious period

The time people are usually most infectious are the first 3 days after symptoms appear. You can infect others from the day before you show symptoms until around 7 days after symptoms begin, sometimes longer.

Prevention

Vaccination

Get vaccinated

The best ways to protect against seasonal flu is to get vaccinated each year, especially around April or May.

Flu vaccines are available to protect against the most common strains. Eligible people can get vaccines for free under the National Immunisation Program or state or territory programs. If you are not eligible for a free vaccine, you can buy one from your health professional or pharmacy.

Other prevention steps

Other ways to protect against seasonal flu are to:

  • if you are sick with cold and flu symptoms, stay home and if you need to leave home, wear a mask
  • put physical distance between yourself and others
  • maintain good hygiene, including by regularly washing your hands and maintaining respiratory hygiene
  • avoid touching your eyes, nose and mouth, particularly if you are around people who are unwell
  • regularly clean objects and surfaces you use often, such as mobile phones and door handles.

Read more ways to prevent the flu.

Priority groups and settings

Some people are at greater risk of getting the flu or of getting very sick from it.

The flu can spread quickly in homes and other settings including:

  • residential aged care homes
  • other residential care facilities
  • childcare centres
  • schools. 

If you are at greater risk, it is especially important to: 

  • get your flu vaccination each year – it’s free for certain people in high-risk groups
  • take steps to protect yourself
  • avoid crowded places
  • avoid people who are sick with cold and flu symptoms
  • see your health professional as soon as symptoms start – medicine works best when taken in the first 2 days of illness.

People at greater risk may choose to wear a face mask in a public place, and when around others. 

People at greater risk of severe disease

Seasonal flu poses a greater risk of severe illness for:

  • babies and young children (aged under 5 years)
  • people aged 65 years or older
  • people who are pregnant
  • Aboriginal and Torres Strait Islander people
  • people with some chronic conditions
  • people who are immunocompromised
  • people who smoke.

People at greater risk of becoming infected

People are at greater risk of becoming infected with seasonal flu if they: 

  • are not up to date with their annual vaccination
  • have had contact with someone who has the flu – especially if they have been in close and prolonged contact (such as living in the same house).

Diagnosis and treatment

Your health professional can diagnose the flu with a flu test. The sample is usually collected from your nose or throat with a swab and sent do a laboratory for testing.

You can test yourself with a rapid antigen test (RAT) that detects the flu. These tests are widely available in supermarkets and pharmacies. These are not as accurate as tests requested by your health professional.

Read more about diagnosing the flu.

You can use healthdirect’s directory to find a health service near you.

Find a health service

Most people will get better without any treatment within 7 to 10 days. If you’re at risk of severe illness, your health professional might prescribe antiviral or pain medicines. 

Antibiotics are not used to treat the flu, because they do not work for viral infections. Taking antibiotics when you don’t need them can increase the risk of antibiotic resistance.

Read more about how to manage or treat the flu.

Surveillance and reporting

Influenza is a nationally notifiable disease – these are diseases that present a risk to public health.

Health authorities in each state and territory report new laboratory confirmed cases to us daily through the National Notifiable Diseases Surveillance System. 

We also track data on flu collected through other surveillance systems, including:

This is part of our surveillance activities, which help us monitor case numbers around the country and understand disease patterns. 

We analyse the data, and report on flu case numbers through:

The Australian national surveillance plan for COVID-19, influenza, and RSV guides how we collect, analyse and report on health-related data for COVID-19.

Read more about our influenza surveillance.

Outbreaks

Seasonal flu outbreaks are common, especially in winter months. Their frequency and severity vary depending on:

  • the strains circulating
  • vaccination rate
  • public health measures.

State and territory health departments provide guidance and support for outbreaks of seasonal flu – for example, in aged care homes or hospitals. 

We have a national plan for responding to flu in certain situations when it presents a serious threat to Australia, such as when: 

  • a new flu virus with pandemic potential is circulating
  • a state or territory requests assistance with seasonal flu. 

In these cases, we follow our action plans, to minimise the impact on the health of Australians and on our health system. 

Read more about how the Australian Government defines and plans for outbreaks and pandemics.

Support

For information about seasonal flu in your state or territory see:

Quick links

Disease

Infectious agent

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:

Clinical presentation

Influenza illness ranges from asymptomatic infection to severe disease. 

Clinical features typically include:

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. 

Epidemiology

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:

Spread of infection

Transmission

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.[1]

Incubation period

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

Infectious period

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

People at greater risk of severe disease

Seasonal influenza poses a greater risk of severe illness for:

  • babies and young children (under 5 years old)
  • older people (aged 65 years or older)
  • people who are pregnant
  • Aboriginal and Torres Strait Islander people
  • people with certain chronic conditions
  • people who are immunocompromised
  • people who smoke.

Read more about:

People at greater risk of becoming infected

People are at greater risk of becoming infected with seasonal influenza if they: 

  • are not up to date with their annual vaccination
  • have had contact with someone who has influenza – especially if they have been in close and prolonged contact (such as living in the same house).

See what measures people can take to protect themselves.

Settings at increased risk of spread

Influenza spreads quickly in crowded places and some settings, including: 

  • residential care facilities, including residential aged care homes
  • early childhood care centres
  • schools. 

These settings should actively promote measures people can take to protect themselves, including by:

  • encouraging annual vaccination of staff, residents and students
  • ensuring unwell staff – and children in those settings – stay home.

An influenza outbreak in these settings may require a public health response. 

Read more about the prevention, control and public health management of outbreaks in residential aged care homes.

Prevention

Vaccination

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

Other prevention steps

Other prevention measures include good hygiene practices, including: 

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. 

Prevention among priority populations

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

Diagnosis

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.

Laboratory case definition

Influenza – Laboratory case definition

The Public Health Laboratory Network (PHLN) has developed standard case definitions for the diagnosis of key diseases in Australia. This document contains the laboratory case definition for influenza.

 

Clinical management

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

Surveillance case definition

 

National notification and reporting

Influenza is a nationally notifiable disease. This means certain health professionals must report diagnoses through their relevant health authorities.

Check how to report laboratory-confirmed influenza cases or related death in your state or territory:

State and territory health authorities report new cases to us daily through the National Notifiable Diseases Surveillance System, as part of our surveillance activities.

We report case numbers and activity through our: 

We also track influenza cases through other surveillance systems, including:

This helps us to:

  • identify trends in influenza activity and impact
  • assess the impact of influenza control programs
  • develop policies to minimise the impact of influenza.

Public health response

National public health guidelines

 

Management of cases

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.

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

Management of contacts

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

Outbreak response

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.  

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

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

Resources

See:

References

  • 1 JS Greatorex, P Digard et al., 'Survival of influenza A (H1N1) on materials found in households: Implications for infection control', PLOS ONE 2011, 6(11):e27932, doi:10.1371/journal.pone.0027932.
  • 2 JS Bresee, AM Fry et al., 'Inactivated influenza vaccines', Plotkin's Vaccines (7th edition), 2018, e21:456–488, doi:10.1016/B978-0-323-35761-6.00031-6.

Latest resources

National Notifiable Diseases Surveillance System (NNDSS) public datasets

This collection contains National Notifiable Diseases Surveillance System (NNDSS) datasets for influenza, meningococcal disease, pneumococcal disease and salmonellosis. We update these in July of each year to include the previous year's notifications and all updates.

NNDSS public dataset – influenza (laboratory confirmed)

This National Notifiable Diseases Surveillance System (NNDSS) dataset provides records of laboratory-confirmed influenza cases from 2008, by month and other patient variables. We update it in July of each year to include the previous year's notifications and all updates.

Influenza – Laboratory case definition

The Public Health Laboratory Network (PHLN) has developed standard case definitions for the diagnosis of key diseases in Australia. This document contains the laboratory case definition for influenza.
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