Influenza

Notification requirement for influenza

As of 1 January 2023, pathology services must also notify notification details for all tests performed during a weekly period in relation to influenza. This notification must be in writing within 5 business days of the end of that period.

Influenza is a ‘routine’ notifiable condition and must be notified by pathology services in writing within 5 days of laboratory confirmation. This is a Victorian statutory requirement.

Medical practitioners are not required to notify cases of influenza.

Primary school and children’s services centre exclusion for influenza

Exclusion from school and childcare is recommended until symptoms resolve.

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

• cough
• headache
• myalgia
• lethargy
• coryza
• sore throat.

Most symptoms resolve within 2 to 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.

Severe disease and complications occur primarily among the elderly and those debilitated by a chronic disease, including:

• diabetes
• cardiac disease
• chronic respiratory conditions.

Other people at increased risk of severe disease include:

• Aboriginal or Torres Strait Islander people
• pregnant women
• children under 5 years of age.

Diagnosis

A clinical diagnosis can be confirmed by

• detection of influenza virus by culture or polymerase chain reaction (PCR) testing of an appropriate respiratory specimen, such as nasopharyngeal aspirate or nose and throat swabs, taken within 5 (preferably 2) days of onset.
• serology performed on blood specimens taken during the acute and convalescent stages. However this provides a retrospective diagnosis and is therefore less useful for clinical or outbreak management.

Incubation period of influenza virus

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

Public health significance and occurrence of influenza

In Victoria, influenza cases peak during the winter months, however infections can occur throughout the year. Influenza activity and disease severity varies from year to year, dependent on the circulating viruses, level of immunity of the population (from vaccination and past infection), and effectiveness of the annual vaccine.

Influenza A and/or B viruses circulate globally causing sporadic cases and epidemics. Influenza C virus occurs less frequently and usually causes mild infections.

The envelopes of influenza viruses contain two surface proteins, hemagglutinin (H) and neuraminidase (N). Influenza A viruses can be classified into subtypes according to combinations of these H and N proteins.

Influenza viruses mutate easily, resulting in antigenic changes in the H and N surface proteins. Gradual changes in the viral antigens (antigenic drift) are responsible for seasonal epidemics of influenza, therefore influenza vaccines need to be formulated each year to counter influenza stains which are most likely to circulate.

An influenza pandemic may occur when there is a significant change in the antigenic makeup of the virus (antigenic shift) resulting in a new highly virulent strain to which there is little or no immunity in the population. Influenza pandemics spread on a worldwide scale and infect a large proportion of the human population.

Seasonal influenza results in an estimated 3,500 deaths each year in Australia, over 300,000 general practitioner consultations and 18,000 hospitalisations.

Population groups at increased risk of severe disease, hospitalisation and mortality include the elderly, children less than 5 years of age, and Aboriginal and Torres Strait Islander peoples.

Reservoir of influenza virus

Humans are the primary reservoir for seasonal influenza. In addition, birds, pigs, and horses are also natural hosts of influenza A virus. 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.

Mode of transmission of influenza virus

Influenza viruses are most commonly spread by inhalation of infectious respiratory droplets produced by an infected person while talking, coughing or sneezing. Influenza can also be spread through touching surfaces where infected droplets have landed. Nasal inoculation after hand contamination with the virus is an important mode of transmission, highlighting the critical importance of hand hygiene.

Period of communicability of influenza

Influenza is communicable for up to 24 hours prior to onset of symptoms, and up to 7 days after onset of symptoms in adults and longer in children (up to 10 days).

Susceptibility and resistance to influenza

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

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

Control measures for influenza

Preventive measures

Annual vaccination is the most important measure to prevent influenza and its complications. The influenza vaccine in Australia is developed in time for the annual winter rise in influenza 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 most recent northern hemisphere winter. The vaccine normally includes both influenza A and B strains.

Vaccination is recommended for everyone from 6 months of age and is free for high-risk groups. For more information on eligibility for the free vaccine, see Seasonal influenza vaccine.

Annual influenza vaccination is also recommended for staff working in high-risk settings such as residential aged care facilities. Healthcare workers who provide direct care to patients are required to have the vaccination to protect themselves and their patients. For more information see Vaccination for healthcare workers.

Additional measure to prevent the spread of influenza include:

• hand hygiene, surface cleaning and correct cough/sneeze etiquette
• use of face masks
• people with influenza-like illness (ILI) should limit use of, or avoid public transport and self-exclude from school, childcare, work or public gatherings
• people with an ILI should not visit family or friends in high-risk settings such as residential care facilities or hospitals.

Control of case

Individual cases are managed by their treating doctor. Symptomatic treatment is indicated alone or with the addition of antiviral medication (i.e. neuraminidase inhibitors). Antiviral medication can reduce the severity and duration of influenza if taken soon after onset of symptoms. Consult the current version of Therapeutic guidelines: antibiotic.

Measures such as hand hygiene, good respiratory and cough etiquette, use of face masks, voluntary home isolation and cleaning of commonly touched surfaces help reduce transmission of influenza. Patients should be advised to avoid contact with people at high risk of developing severe influenza.

Control of contacts

Contacts at high risk of developing severe disease should be advised to seek medical advice on prophylaxis and early medical review if symptoms develop.

Chemoprophylaxis may be considered in people who are at higher risk of influenza complications, and in high-risk settings such as residential aged care facilities.

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 outbreaks is appropriate immunisation.

Public health action focuses on outbreaks in high-risk settings such as healthcare facilities, special schools, residential care facilities (see ‘Special settings’, below).

Special settings

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

Aged and residential care facilities

Prevention in these settings is best achieved by high 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), use of appropriate personal protective equipment, 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.

See detailed guidance for managing respiratory outbreaks in aged care facilities.

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.