Key messages

  • Influenza is a Group B disease that must be notified in writing within 5 days of laboratory confirmation.
  • Seasonal vaccination is recommended for high-risk groups, and is free for some of these groups.
  • An influenza pandemic results when antigenic shift leads to a new, highly virulent influenza subtype to which there is little or no immunity in the population.
  • Residential care facilities, healthcare facilities and childcare centres are all areas at higher risk of influenza outbreaks.

Notification requirement for influenza

Influenza (Group B disease) must be notified in writing within 5 days of laboratory confirmation.

This is a Victorian statutory requirement.

Primary school and children’s services centre exclusion for influenza

Exclude until well. Cases are not excluded unless deemed necessary by the Secretary.

Infectious agent of influenza

Influenza virus (types A, B and rarely C) is the causative agent.

Identification of influenza

Clinical features

Influenza is an acute respiratory disease. Symptoms include fever, headache, myalgia, lethargy, coryza, sore throat and cough. Infections in children may also be associated with gastrointestinal symptoms such as nausea, vomiting and diarrhoea. Croup is a common presentation in children.

Most symptoms resolve within 2–7 days, although the cough may persist for longer. Complications of influenza include middle-ear infections, secondary bacterial pneumonia and exacerbation of underlying chronic health conditions.

During influenza epidemics, patients with early influenza symptoms (fever >38 °C, plus at least one systemic symptom, such as myalgia, and one respiratory symptom) have a 60–70 per cent chance of having influenza infection.


A clinical diagnosis can be confirmed by culture or antigen testing of appropriate respiratory specimens, such as nasopharyngeal aspirate or nose and throat swabs, taken within 5 (preferably 2) days of onset. It can also be confirmed by serology performed on blood specimens taken during the acute and convalescent stages, but this is less useful for clinical or outbreak management.

The diagnosis can be confirmed in the laboratory by one or more of the following:

  • detection of influenza virus by culture or nucleic acid testing, most commonly polymerase chain reaction (PCR) testing
  • demonstration of a significant rise (fourfold increase) in the influenza-specific antibody titre between a serum sample collected in the acute phase and another sample collected in the convalescent phase 2–3 weeks after onset of symptoms
  • a single high influenza-specific antibody titre of five dilutions or greater, depending on the titration method; this means a titre of 160 or greater, or 128 or greater.

Incubation period of influenza virus

The incubation period is 1–4 days, with an average of 2 days for seasonal influenza.

Public health significance and occurrence of influenza

Influenza occurs as pandemics, epidemics and sporadic/seasonal cases. An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the human population. This occurs as a result of significant change in the antigenic makeup of the virus, usually through sudden antigenic shift (reassortment) and the emergence of an entirely new subtype. Antigenic drift is a gradual change in the viral antigens, and is responsible for seasonal epidemics and regional outbreaks.

Severe disease and complications such as viral pneumonitis and bacterial pneumonia occur primarily among the elderly and those debilitated by a chronic disease, including diabetes, cardiac disease and chronic respiratory conditions. Other people at increased risk of severe disease include Aboriginal or Torres Strait Islander people older than 15 years, pregnant women, and children under 5 years of age. In the 2009 pandemic, people with morbid obesity (body mass index > 40) were also found to be at a higher risk of complications.

In temperate zones, outbreaks tend to occur in winter. In the tropics, they often occur in the rainy season, but outbreaks or sporadic cases may occur at any time.

Most human infections are caused by either type A or type B influenza viruses. Type A has been associated with widespread epidemics and pandemics, while type B has been infrequently associated with regional epidemics, and type C is only rarely associated with human infection.

Influenza A is subtyped further. The virus has two surface antigens (proteins) that are used for subtyping: haemagglutinin (H) and neuraminidase (N). Since 1918, the three influenza A subtypes that usually cause human disease are H1N1, H2N2 and H3N2. Other subtypes such as H5N1 are very rare.

Influenza viruses are named according to type (A, B or C), subtype and antigenic characterisation, including year of isolation. Only type A viruses are subtyped – for example, influenza A (H1N1)/New Caledonia/20/99.

Reservoir of influenza virus

Humans are the primary reservoir. Animal reservoirs are suspected as sources of new human subtypes, and may occur particularly when people and livestock (for example, pigs and poultry) interact closely. In 2004, an outbreak of avian influenza (influenza A H5N1) caused a number of human infections in South-East Asia, and continues to cause outbreaks in birds and sporadic human cases, especially in Egypt and Indonesia.

Mode of transmission of influenza virus

Influenza viruses are predominantly transmitted by airborne spread in aerosols, but can also be transferred by direct contact with droplets. Nasal inoculation after hand contamination with the virus is also an important mode of transmission, highlighting the critical importance of hand hygiene.

Direct contact is important, as the virus will survive some hours in dried mucus, particularly in cold and dry environments.

Period of communicability of influenza

Influenza is probably communicable for 3–5 days from clinical onset in adults, and up to 7 days and occasionally longer in young children.

Susceptibility and resistance to influenza

When a new subtype appears, all people are susceptible, except those who have lived through earlier epidemics or pandemics caused by a related subtype.

Infection produces immunity to the specific infecting virus, but the duration and breadth of immunity vary widely. This is partly dependent on host factors, the degree of antigenic drift in the virus and the period of time since the previous infection.

Control measures for influenza

Preventive measures

The influenza vaccine in Australia is developed in time for the annual winter rise in ‘flu’ activity. The strains that are contained in the vaccine are ‘best guess’, based on the circulating strains in the previous couple of years, as well as those circulating in the previous Northern Hemisphere winter. The vaccine normally includes representatives of both major influenza A subtypes (H1N1, H3N2) and B strain.

Free annual influenza vaccine is provided and recommended for the following groups in Victoria:

  • people aged 65 years and older
  • pregnant women at any stage of pregnancy
  • Aboriginal and Torres Strait Islander people aged 6 months to 4 years of age inclusive, and 15 years and older
  • residents of nursing homes and other long-term care facilities
  • those aged 6 months or older with conditions predisposing to severe illness following influenza infection.

Annual influenza vaccination is also recommended for staff working in nursing homes and other chronic care facilities, to protect themselves and their patients. Hospital staff in both outpatient and ward settings who provide direct care to patients are strongly encouraged to have the vaccination to protect themselves and their patients.

Control of case

Symptomatic treatment alone or with the addition of a neuraminidase inhibitor, if commenced within the first 36 hours of the onset of the illness, can shorten the duration by 2–3 days. Consult the current version of Therapeutic guidelines: antibiotic.

For sporadic cases, isolation is often unrealistic because of the delay in diagnosis. If cases are still symptomatic, they should be advised to remain at home until well and to avoid contact with high-risk people.

Control of contacts

Control of contacts may be of benefit in high-risk populations, who should be advised to seek medical advice on prophylaxis and to seek early medical review if symptoms develop.

Chemoprophylaxis with amantadine or a neuraminidase inhibitor may be considered in special circumstances against influenza A strains – for example, in residential institutions. The potential value of chemoprophylactic drugs must be assessed against their side effects.

Control of environment

Cases and carers should be advised about the importance of handwashing, covering the mouth when coughing, sneezing into disposable tissues, and appropriate cleaning or disposal of contaminated objects.

Outbreak measures for influenza

The most important control measure to prevent and control influenza epidemics is appropriate immunisation. Investigations are generally restricted to outbreaks in groups at higher risk of complications (see ‘Special settings’, below).

An influenza pandemic results when antigenic shift leads to a new, highly virulent influenza subtype to which there is little or no immunity in the population. Public health action in this setting may involve a variety of measures to control spread in the community.

Special settings

Aged and other residential care facilities, healthcare facilities and childcare centres are all special areas at higher risk of influenza outbreaks.

Detailed information for these and other special settings in the event of an influenza pandemic can be found in Appendixes 11–14 of the Victorian health management plan for pandemic influenza.

Aged and residential care facilities

Prevention in these settings is best achieved by the highest possible rates of vaccination of both residents and carers.

Specific infection control measures should be implemented in the event of:

  • a laboratory-confirmed case of influenza
  • two or more cases of an acute respiratory illness consistent with influenza (‘influenza-like illness’).

Infection control measures include cleaning of surfaces (especially high-touch surfaces), exclusion of sick staff members, nursing of cases by immunised staff, cohorting of resident cases, active case finding, reduced admissions and transfers, and, in some settings, the use of antiviral treatment and prophylaxis.

Healthcare facilities

Outbreaks of an unidentified respiratory illness in a hospital setting, including outbreaks of influenza-like illness, are investigated and managed jointly by the department and the hospital’s infection control unit.

Childcare centres

Outbreaks of influenza or influenza-like illness in childcare require exclusion of cases and may warrant prophylaxis for high-risk contacts. The department can advise on prophylaxis and infection control procedures.

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