Influenza (flu)

What

Influenza is a common disease of the respiratory tract. It affects people of all ages.

Who

Annual influenza vaccination is recommended for everyone ≥6 months of age.

Influenza vaccination is particularly recommended for:

• children aged 6 months to <5 years
• adults aged ≥65 years
• Aboriginal and Torres Strait Islander people
• people with medical conditions that increase their risk of severe influenza
• pregnant women
• healthcare workers, carers and household contacts of people in high-risk groups
• residents, staff, volunteers and visitors to aged care and long-term residential facilities
• commercial poultry and pork industry workers
• essential services providers
• people who are travelling during influenza season
• people experiencing homelessness

People with the following medical conditions have a higher risk of severe influenza:

• cardiac disease
• chronic respiratory conditions
• immunocompromising condition, such as HIV, malignancy, asplenia, receving a haematopoietic stem cell or solid organ transplant etc.
• haematological disorder
• chronic metabolic disorder
• chronic kidney disease
• chronic neurologic conditions
• long-term aspirin therapy in children (aged 6 months to 10 years)
• chronic liver disease
• obesity
• chromosomal abnormality
• harmful use of alcohol

Also see People at risk of severe disease from influenza.

How

People are recommended to receive an influenza vaccine every year.

Most people should receive 1 dose of influenza vaccine each year. However, the following people should receive 2 doses, given 4 weeks apart:

• children aged 6 months to <2 years receiving influenza vaccine for the first time
• children aged 6 months to <9 years with a medical risk condition receiving influenza vaccine for the first time
• people of any age receiving influenza vaccine for the first time after haematopoietic stem cell or solid organ transplant or CAR T-cell therapy.

The type of vaccine used depends on the person’s age:

• People aged 6 months to <50 years should receive a standard-dose influenza vaccine, either egg-based or cell-based.
• Children aged 2 to <18 years may receive either intramuscularly-administered inactivated influenza vaccine or intranasally-administered live attenuated influenza vaccine (LAIV), except where contraindicated.
• People aged 50 to <65 years may receive either standard-dose or adjuvanted influenza vaccine.
• People aged ≥65 years should receive adjuvanted (NIP-funded) or high-dose influenza vaccine, but may receive standard-dose influenza vaccine if the adjuvanted or high-dose vaccine is unavailable.

Why

Influenza is the most common vaccine-preventable disease in Australia. Although it can be a mild disease, it can also cause very serious illness in otherwise healthy people. It can require hospitalisation and can cause death.

Influenza vaccines available in Australia

Most influenza vaccines available in Australia are either split virion or subunit vaccines. They are prepared from purified inactivated influenza virus that has been cultivated in embryonated hens’ eggs (e.g standard-dose, high-dose and adjuvanted influenza vaccine) or propagated in Madin-Darby canine kidney (MDCK) cells (cell-based influenza vaccine). All influenza vaccines are inactivated vaccines, with the exception of FluMist, which is a live attenuated influenza vaccine (LAIV).

All vaccines are trivalent as they contain 3 influenza virus strains – 2 influenza A subtypes and 1 influenza B lineage.

Standard-dose egg-based inactivated influenza vaccines contain 15 µg of haemagglutinin per strain per dose with no adjuvant. Similarly, the available cell-based influenza vaccine contains the standard dose of 15 µg of haemagglutinin per strain per dose with no adjuvant. Each 0.2 mL dose of LAIV contains 10^6.5-7.5 fluorescent focus units of influenza virus type A (H1N1), type A (H3N2) and type B (Victoria) strains.

The available adjuvanted influenza vaccine is egg-based, and contains the standard 15 µg of haemagglutinin per strain per dose, with MF59 as the adjuvant. The adjuvanted influenza vaccine is formulated to induce a greater immune response than standard-dose influenza vaccines.

High-Dose (egg-based) influenza vaccines contain 60 µg of haemagglutinin per strain per dose with no adjuvant.

Although egg-based vaccines may contain traces of egg-derived protein (ovalbumin), they contain less than 1 μg of ovalbumin per dose. See also Contraindications and precautions, and Vaccinating people with a known egg allergy in Vaccination for people who have had an adverse event following immunisation.

The Therapeutic Goods Administration website provides product information for each vaccine.

See also Vaccine information and Variations from product information for more details.

Always check annual seasonal influenza vaccine statements. Vaccines and age eligibility change from year to year.

Annual changes to influenza vaccines

Influenza vaccines can change from year to year with regard to:

• which vaccines are registered by the Therapeutic Goods Administration
• the indicated ages for each vaccine

Always check annual seasonal influenza statements published by the Australian Technical Advisory Group on Immunisation on the health.gov.au website as well as any state-based recommendations and program details and consult the product information for each vaccine.

Dose and route

Inactivated influenza vaccines available in Australia come in pre-filled syringes of 0.5 mL.

Shake the pre-filled syringe before injection. Inactivated influenza vaccines are preferably given by intramuscular injection. However, they may also be given subcutaneously (refer to Administration of vaccines).

The live attenuated influenza vaccine (LAIV) FluMist is intranasally-administered and not injected. Each 0.2 mL dose is administered by quickly spraying approximately one-half of the contents of the sprayer into each nostril. See Intranasal administration technique in Administration of vaccines.

Doses for children <2 years of age

Healthy children aged 6 months to <2 years receiving influenza vaccine for the first time are recommended 2 doses given at least 4 weeks apart. This maximises the immune response to the vaccine strains. These children should only receive inactivated influenza vaccine.

Children who have received 1 or more doses of influenza vaccine in a previous season only need 1 dose of influenza vaccine in the current and future seasons.^52,53 Influenza vaccine effectiveness is higher among children aged <2 years receiving influenza vaccine for the first time if they are given two doses, whereas additional protection was not observed from 2 doses for older children.⁵⁴

Follow standard recommendations when giving any extra dose(s) during the current or future seasons (see Table. Recommended doses of influenza vaccine and vaccine type by age group).

Doses for children ≥2 years and adults

People aged ≥2 years need 1 dose of influenza vaccine every year, regardless of whether they have ever had influenza vaccine before. These children can receive either inactivated influenza vaccine or LAIV.

The exception is children aged <9 years who have a medical at-risk condition who have not previously received influenza vaccine. These children should receive 2 doses of influenza vaccine, given 4 weeks apart, when vaccinated for the first time.

Follow standard recommendations when giving any extra dose(s) during the current or future seasons (see Table. Recommended doses of influenza vaccine and vaccine type by age group).

Co-administration with other vaccines

People can receive influenza vaccines (either inactivated or LAIV) at the same time as, or separate to, most other vaccines, including dTpa, RSV, pneumococcal and COVID-19 vaccines.

LAIV can be given at the same time as, or at any interval before or after other currently used vaccines, including live vaccines. Studies of LAIV co-administered with MMR, varicella and oral polio vaccines show similar immune responses and safety profiles as when they are administered separately.⁵⁵ People can receive influenza vaccines, including the adjuvanted vaccines Fluad with Shingrix, either at the same time, or at any time before and/or after. There is a potential for an increase in mild to moderate adverse events when more than one vaccine is given at the same time. Vaccine safety surveillance data from Australia reports similar rates of adverse events following Shingrix when co-administered with other vaccines as compared to Shingrix administered alone.

Co-administration of influenza and RSV vaccines predominantly show non-inferior immune responses to influenza or RSV vaccines, with the exception of concomitant administration of Arexvy (RSV vaccine) with adjuvanted influenza vaccine where slightly lower immunogenicity was seen with one of 4 influenza vaccine strains (A/H3N2).^56-58  This lower immunogenicity was not seen in co-administration studies with unadjuvanted quadrivalent influenza vaccine.^56,58,59 The clinical significance of the lower immunogenicity is not known but is unlikely to have a major impact on protection against this strain.

An increase in local and systemic adverse events with co-administration is possible, based on safety data regarding concomitant administration of RSV and influenza vaccines in older adults.^56,58 Data on co-administration with RSV vaccines in pregnant women is still emerging, however there are no theoretical concerns. Benefits of co-administration should be weighed against the potential for increased local and systemic adverse events.

Interchangeability of influenza vaccines

People who require 2 doses of influenza vaccine within a single season may receive different vaccines for each dose, provided that both doses are with an age-appropriate vaccine that is not contraindicated. See Table. Recommended doses of influenza vaccine and vaccine type by age group.

Interchangeability is permitted across different vaccine brands, types (e.g. cell-based or egg-based) and routes of administration (e.g. intranasal or intramuscular).

Timing of influenza vaccination

Annual influenza vaccination is recommended before the influenza season starts. Influenza circulation usually peaks between June and September in most parts of Australia. However, influenza can occur year-round.

Protection is expected to last for the whole season, but optimal protection is within the first 3–4 months after vaccination. Deferring vaccination to the beginning of winter may result in greater immunity later in the season, but may also result in missed opportunities for vaccination and lack of protection if the influenza season starts early.

Immunisation providers need to weigh up these factors for each person and balance them with the challenge of vaccinating large numbers of people in a short time.

Offer vaccination throughout the influenza season. It is never too late to vaccinate, because influenza can circulate all year. In particular, pregnant women and travellers can benefit from vaccination at any time of the year.

Children aged 6 months to <2 years and children aged 6 months to <9 years for those with medical conditions that increase their risk of influenza who are receiving their 1st lifetime dose should receive the vaccine as soon as possible after it becomes available. This helps to ensure enough time to receive a 2nd dose (recommended ≥4 weeks later) before the influenza season starts.

Following the COVID-19 pandemic there was a resurgence of influenza virus circulation with the reopening of international borders. The importance of influenza vaccination should continue to be emphasised.

If a person had a 2025 influenza vaccine in late 2025 or early 2026, they are still recommended to receive a 2026 formulation of influenza vaccine.

Contraindications

The only absolute contraindications to influenza vaccines are:

• anaphylaxis after a previous dose of any influenza vaccine
• anaphylaxis after any component of an influenza vaccine

Additionally, LAIV is contraindicated in:

• people who are moderately and severely immunocompromised (see Vaccination for people who are immunocompromised)
• people receiving oral salicylate therapy (e.g. Aspirin)

Precautions

People with egg allergy

None of the available influenza vaccines contain >1 µg of ovalbumin.

People with egg allergy, including a history of anaphylaxis, can be safely vaccinated with any influenza vaccines (including egg-based, cell-based and live attenuated vaccines ) unless they have reported a serious adverse reaction to influenza vaccines. See also Vaccinating people with a known egg allergy in Vaccination for people who have had an adverse event following immunisation.

People with known anaphylaxis egg allergy

People with a history of anaphylaxis to egg should:

• receive a full age-appropriate vaccine dose; do not split the dose into multiple injections (for example, a test and then the rest of the dose)

If there is significant parental or health professional anxiety, the vaccine may be administered in primary care settings with a longer post vaccination observation period of 30 minutes.

Several published reviews, guidelines and reports suggest a very low risk of anaphylaxis associated with influenza vaccination of egg-allergic people.^60-65

A 2012 review of published studies included 4172 patients with a history of egg allergy, including 513 patients with a history of severe allergy. The review found no cases of anaphylaxis after receiving an inactivated influenza vaccine.⁶⁴

The largest study in the review included 830 egg-allergic patients. 164 of these patients reported a history of severe allergic reaction to egg. Only 17 (2%) of these patients experienced any adverse event.⁶⁶ All adverse events were mild, and included abdominal pain, hives and respiratory symptoms such as wheezing.

People with known non-anaphylaxis egg allergy

People with a history of egg allergy (non-anaphylaxis) can receive an age-appropriate full dose of vaccine in any immunisation setting. This includes sensitised children (that is, children who are skin-prick or RAST-test positive) who have not yet eaten egg.

Women who are pregnant or planning pregnancy

All inactivated influenza vaccines can safely be given to pregnant and breastfeeding women. 

Pregnant women should receive inactivated influenza vaccines rather than live attenuated influenza vaccines (LAIV). However, there is no evidence of risk associated with LAIV given in pregnancy. A large 2011 study from the United States found no evidence of adverse maternal outcomes and all adverse outcomes identified occurred at similar rates to those in unvaccinated women.⁶⁷ Data from case reports and spontaneous reports to the United States Vaccine Adverse Event Reporting System (VAERS) have not identified any increased risk of adverse fetal outcomes following LAIV administration to pregnant women.^67,68 

The live viruses in LAIV have been attenuated and adapted to cold, allowing them to only replicate at the lower temperatures found in the nasal passage. They cannot replicate efficiently anywhere else in the body and there is therefore no theoretical basis for concern about infection of the unborn foetus or the mother’s lungs. It is not necessary to specifically ask about or test for pregnancy when offering LAIV or to advise avoidance of pregnancy in those who have been recently vaccinated. 

People with a history of Guillain–Barré syndrome

People with a history of Guillain–Barré syndrome (GBS) whose first episode was not after influenza vaccination have an extremely low risk of recurrence of GBS after vaccination. Influenza vaccination is recommended for these people.^69-71

People with a history of GBS whose episode occurred within 6 weeks of receiving a vaccine may have a higher risk of GBS recurrence than if the previous GBS episode was unrelated to vaccination. They should discuss with their provider the risks and benefits of influenza vaccination, particularly if their episode was related to a vaccination. 

People living or working in close contact with severely immunocompromised people

As a precaution, use of IIV is preferred over LAIV in people who are living with or working in close contact with someone who is severely immunocompromised. Exposure of pregnant women to people who have received LAIV is less of a concern and should not be a reason to avoid vaccinating another person in close contact with a pregnant woman. The risk of transmission attributable to shedding during vaccine administration in a mass vaccination setting has not been formally assessed, but is expected to be low.

People receiving immuno-oncology therapy

Some studies that included a small number of patients reported that people receiving cancer immuno-oncology therapies (checkpoint inhibitors) may have a higher risk of immune-related adverse events following immunisation with influenza vaccine,^72,73 but a more recent study on patients receiving treatment with a single checkpoint inhibitor did not.^74-77

Checkpoint inhibitors include:

• CTLA-4 inhibitors (such as ipilimumab)
• PD-1 and PD-L1 inhibitors (such as nivolumab or pembrolizumab)

Consult the person’s treating oncologist about the risks and benefits of influenza vaccination in people taking multiple immune checkpoint inhibitors.

Children with cochlear implants

Children with cochlear implants can receive LAIV.

If possible, LAIV should be administered at least one week before cochlear implant surgery. Alternatively, it can be given 2 weeks after the surgery, when the surgical site has healed and there is no active infection or evidence of ongoing cerebrospinal fluid leak.

Post-vaccination symptoms following any influenza vaccine may mimic influenza infection. Influenza vaccines do not cause influenza. Even LAIV, which contains weakened live virus, cannot cause influenza disease.

Injection site reactions after standard-dose inactivated influenza vaccines

Injection site reactions occur in more than 10% of people who receive standard-dose inactivated influenza vaccine intramuscularly. These include:^78-80

• induration
• swelling
• redness
• pain

Systemic adverse events after standard-dose inactivated influenza vaccines

1–10% of people who receive standard-dose inactivated influenza vaccines will develop:^78-83

• fever
• malaise
• myalgia

These adverse events may start a few hours after vaccination and may last for 1–2 days.^78,79,81

Immediate adverse events following influenza vaccination are very rare. They can include hives, angioedema or anaphylaxis.^60,65

People with a history of anaphylaxis after eating eggs or a history of a severe allergic reaction after occupational exposure to egg protein may receive influenza vaccination.^60,65 See People with egg allergy in Contraindications and precautions.

Adverse events after LAIV

As LAIV is administered intranasally, it can cause upper respiratory and nasal symptoms, like runny nose, nasal congestion or sore throat. Data from large clinical trials show that children who received LAIV have an increased odds of experiencing nasal symptoms relative to children who received IIV (OR=1.64; 95%CI: 1.33–20.2).⁸⁴

There is no difference between LAIV and inactivated influenza vaccine in regards to serious adverse events. 

LAIV is safe in children with asthma/wheeze. Early studies of LAIV raised concerns about wheezing in young children and possible asthma exacerbation. An early trial found a higher rate of “reactive airway” illness in children aged 18–35 months, and another showed an increase in wheezing among unvaccinated children aged 6–23 months compared with IIV.⁸⁵ However, numerous studies since then – including a 2025 review of 24 studies involving more than 1.2 million people – have confirmed that LAIV and IIV have comparable safety for children with asthma or recurrent wheeze.^86-92

In studies among children who were mildly immunocompromised, specifically children with cystic fibrosis, HIV infection and those receiving cancer treatment, LAIV vaccination was well tolerated; while some mild adverse events occurred more frequently relative to healthy children (mostly nasal symptoms), vaccine-related serious adverse events were not increased.^93-95 These studies also demonstrated absence of prolonged shedding in children with mildly immunocompromising conditions.

Adverse events after adjuvanted influenza vaccine

Clinical trials show a higher rate of injection site reactions in adults aged ≥50 years after receiving the adjuvanted influenza vaccine, compared with standard-dose influenza vaccines.

Around 30% of people who received Fluad reported injection site reactions, compared with around 20% of people who received standard-dose influenza vaccine. Both groups reported similar rates (about 30%) of systemic reactions.⁹⁶ Rates of severe or serious adverse events do not increase after receiving the adjuvanted vaccine. This has been shown in clinical trials and post-licensure surveillance studies.^96-100

Adjuvanted influenza vaccine is only registered for use in people aged ≥50 years. This vaccine is not recommended in younger people. However, clinical trials in some younger populations and post-licensure safety data after an adjuvanted vaccine was inadvertently given to younger people suggest a similar safety profile to that seen in elderly people.^101-103

Adverse events after High-Dose influenza vaccine

Clinical trials show a higher rate of mild to moderate injection site reactions in adults aged ≥65 years after receiving the High-Dose influenza vaccine, compared with standard-dose influenza vaccines. There were no differences in the frequency of severe local adverse events.

Some studies reported a slightly higher frequency of systemic events among High-Dose influenza vaccine recipients compared with standard-dose influenza vaccines recipients, however events were mostly mild-moderate.

In a large active surveillance study, the frequency of any adverse event was 8.9% among High-Dose influenza vaccine recipients compared with 6.3% among standard-dose egg-based influenza vaccine recipients.¹⁰⁴ However the frequency who sought medical attention was similar between the groups.

Adverse events after cell-based influenza vaccine

Cell-based influenza vaccines have a similar safety profile to standard-dose egg-based influenza vaccines. In a study of children aged 6 months through 4 years, rates of adverse events were similar between those who received cell-based and egg-based vaccines with no serious adverse events related to vaccination.¹⁰⁵ In study among children and adolescents aged 4–17 years, injection site reactions were reported in 53% of people receiving cell-based vaccine compared with 43% receiving standard-dose egg-based influenza vaccine. Systemic reactions were reported by 37% and 30%, respectively.¹⁰⁶ Both injection site and systemic reactions were typically mild to moderate; <1% were reported as severe.

In another study in adults aged 18–60 years, injection site reactions were reported in 29% of people receiving cell-based vaccine compared with 25% receiving standard-dose egg-based influenza vaccine. Systemic reactions were reported by 25% and 23%, respectively.¹⁰⁷ Injection site reactions were typically mild to moderate; <1% were reported as severe. No severe systemic reactions were reported.

Fever and febrile convulsions in children aged <5 years

In 2010, higher rates of fever and febrile convulsions were reported in children aged <5 years after influenza vaccination, especially in children aged <3 years.

Only the Seqirus (previously bioCSL) vaccines Fluvax and Fluvax Junior were associated with this side effect. After vaccination with Fluvax or Fluvax Junior, children <5 years of age had febrile convulsions at a rate of 4.4 per 1000 doses, compared with no such events reported among children who received an alternative vaccine in the same year.¹⁰⁸

The Fluvax and Fluvax Junior vaccines are no longer available in Australia and available Seqirus vaccines have been reformulated.

Safety of influenza vaccine during pregnancy or breastfeeding

Inactivated influenza vaccine is safe to give during any stage of pregnancy or while breastfeeding for both the mother and her baby.^68,109-111 Several systematic reviews have shown no association between influenza vaccination in pregnancy and adverse birth outcomes.^112,113 

Inactivated influenza vaccines should be used in pregnancy instead of LAIV. There is, however, no evidence of risk associated with LAIV if it is inadvertently administered during pregnancy. A large 2011 study from the United States found no evidence of adverse maternal outcomes and all adverse outcomes identified occurred at similar rates to those in unvaccinated women.⁶⁷ Data from case reports and spontaneous reports to the United States Vaccine Adverse Event Reporting System (VAERS) have not identified any increased risk of adverse fetal outcomes following LAIV administration to pregnant women.^67,68

Guillain–Barré syndrome

In 1976, a small increased risk of Guillain–Barré syndrome (GBS) was associated with a specific formulation of influenza vaccine in the United States. Since then, close surveillance has shown that GBS occurs at a very low rate of up to 1 in 1 million doses of influenza vaccine.¹¹⁴ The risk of GBS following influenza illness is estimated to be much higher to about 15 times the risk of GBS following influenza vaccine.^115,116 If GBS is suspected, it is preferable that an experienced clinician confirms the diagnosis so recommendations can be made regarding future influenza vaccination.

See also People with a history of Guillain–Barré syndrome, and Uncommon and rare adverse events following immunisation in After vaccination.

Narcolepsy

Narcolepsy (sudden sleeping illness) has been associated with AS03-adjuvanted pandemic influenza vaccines. This was mainly seen in the Scandinavian population, and affected children especially.^117-119 These vaccines were not used, and have never been available, in Australia.

The only registered adjuvanted influenza vaccine in Australia contains a different adjuvant (squalene-based MF59 oil-in-water emulsion adjuvant). Narcolepsy has not been associated with influenza vaccine containing MF59 adjuvant, although the number of subjects aged <20 years in these studies was limited.^120,121

Influenza viruses are single-stranded RNA orthomyxoviruses. They are classified antigenically as types A, B, C or D. Generally, only influenza A and B cause severe disease in humans.¹²²

Surface glycoprotein antigens

Influenza viruses have 2 surface glycoprotein antigens:

• haemagglutinin (H), which is involved in cell attachment during infection
• neuraminidase (N), which facilitates the release of newly synthesised virus from the cell

Influenza A viruses can be classified into subtypes based on differences in these surface antigens, but influenza B viruses cannot. There are 2 distinct influenza B lineages that previously co-circulated in varying proportions from year to year: B/Victoria and B/Yamagata.^123,124However, B/Yamagata strain has not been detected in global circulation since 2020.¹²⁵

Antibodies against the surface antigens, particularly haemagglutinin, reduce infection or severe illness due to influenza.

Antigenic drift and seasonal influenza

The surface antigens of influenza A and influenza B viruses change often. The changes involve stepwise mutations of genes coding for H and N glycoproteins. This results in cumulative changes in influenza antigens, called antigenic drift. Antigenic drift is responsible for the annual outbreaks and epidemics of influenza. The composition of influenza vaccines needs to be reviewed every year because of antigenic drift.

Antigenic shift and pandemic influenza

Antigenic shift is a dramatic change in the H antigen (and other antigens) of influenza A. This occurs unpredictably and infrequently.¹²²Antigenic shift gives rise to pandemic influenza subtypes by 1 of 2 ways:

• an avian or other animal (such as swine) virus directly adapts so that it can infect humans
• an avian or other animal (such as swine) virus mixes with a human virus, called genetic reassortment

There have been 5 influenza pandemics in the 20th and 21st centuries:

• 1918 (H1N1)
• 1957 (H2N2)
• 1968 (H3N2)
• 1977 (H1N1)
• 2009 (H1N1)

Each of these pandemic strains replaced the previously circulating influenza A subtype and went on to circulate as seasonal influenza.

More recently, various avian influenza A virus subtypes have caused human infections. Examples are H5N1, H7N9 and H9N2. Sustained human-to-human transmission of these subtypes has not been reported.^126,127

Pathogenesis

The incubation period for influenza is usually 2 days, but ranges from 1 to 4 days. Virus can be detected in the upper airway for up to 1 day before symptoms start and around 3–5 days after illness finishes in adults. Children may shed virus for up to 2 weeks. Severely immunocompromised people can shed virus for months.¹²²

Transmission

People transmit influenza from person to person:

• through virus-containing respiratory aerosols produced during coughing or sneezing
• by direct contact with respiratory secretions¹²²

Interspecies transmission (such as from birds to humans) can result in severe illness. In rare cases, transmission of a novel virus into the human population from animals can cause a pandemic.¹²²

Symptoms of influenza

Influenza virus infection causes a wide spectrum of disease. People may have: 

• no or minimal symptoms
• respiratory illness with systemic features
• multisystem complications and death from primary viral or secondary bacterial pneumonia

Symptoms in adults

Fever is an obvious sign of infection and peaks at the height of the systemic illness. Other symptoms are similar for influenza A and B viruses, and include: 

• malaise
• fever
• chills
• headache
• anorexia
• myalgia
• cough
• nasal discharge
• sneezing

Symptoms in children

The clinical features of influenza in infants and children are similar to those in adults. However, children may have: ¹²⁸

• higher fevers
• febrile convulsions
• otitis media
• gastrointestinal symptoms

Infection in young infants may be associated with more non-specific symptoms.^128,129

Complications of influenza

Complications of influenza include:

• acute bronchitis
• croup/laryngotracheobronchitis
• acute otitis media
• pneumonia (primary viral and secondary bacterial pneumonia)
• cardiovascular complications, including myocarditis and pericarditis
• encephalitis and/or encephalopathy
• Reye syndrome
• various haematological abnormalities

Secondary bacterial pneumonia is a common complication in people whose medical condition makes them vulnerable to influenza. These people are at high risk during epidemics, and may die of pneumonia or cardiac decompensation.

People at risk of severe disease from influenza

Severe disease from seasonal influenza is more likely in:^9-29

• older people
• infants
• Indigenous populations (Aboriginal and Torres Strait Islander people in Australia)
• people who have never been exposed to an antigenically related influenza virus
• people who are infected with a highly virulent viral strain
• people with chronic conditions, such as heart or lung disease, renal failure, diabetes and chronic neurologic conditions
• people who are immunocompromised
• people with obesity (BMI ≥30 kg per m²)
• pregnant women
• people who smoke

Severe disease may also occur in otherwise healthy children and young adults. Annual attack rates in the general community are typically 5–10%, but may be up to 20% in some years. In households and ‘closed’ populations, attack rates may be 2–3 times higher.^130,131 However, because asymptomatic or mild influenza illness is common and symptoms are non-specific, many influenza infections are not detected.

Infections due to influenza A (H3N2) strains are more likely to lead to severe morbidity and increased mortality than influenza B or seasonal influenza A (H1N1) strains.^122,132-134

Minor or major epidemics of influenza A or influenza B influenza occur in most years, usually during the winter months in temperate regions. The impact of influenza is often substantially underestimated.

Influenza activity varies from year to year, and depends on the circulating virus and the population’s susceptibility.¹²⁸ Changes in influenza detection methods have affected influenza detection and notification patterns.¹³⁵ This includes an increase in the routine use of PCR-based laboratory testing in recent years.

During annual epidemics, morbidity and mortality are higher among older adults, pregnant women and people with comorbidities.^5,23,136,137

Influenza in Australia

On average each year in Australia, influenza causes approximately 100 deaths and 5100 hospitalisations.⁴ These numbers are widely believed to under-represent the true burden of influenza disease in Australia.

In 2019, an Australian modelling study conducted a comprehensive measurement of influenza disease burden during 2006–2015 in Australia.¹³⁸ In this study, the average excess influenza-associated respiratory mortality rate was 2.61 per 100,000 population, with the highest rates among those aged ≥65 years, at 16.4 per 100,000.¹³⁸ The annual excess influenza-associated hospitalisation rates were similarly highest in adults aged ≥65 years at 195.42 per 100,000 population, followed by children aged <6 months at 153.84 per 100,000 population.¹³⁸

Another mathematical modelling study estimated the annual rate of seasonal influenza-associated mortality to be as high as 26.8 per 100,000 population in Australians aged ≥75 years.¹³⁹ In Australia, as in other high-income countries, older people and children <5 years of age have the highest rates of influenza hospitalisation.^{4,135,140,141} The disease burden from influenza is also greater in Aboriginal and Torres Strait Islander people than in non-Indigenous Australians across all age groups.⁴

During the 2009 A(H1N1)pdm09 pandemic, the predominant clinical presentation was mild to moderate illness. However, young healthy adults, pregnant women, and Aboriginal and Torres Strait Islander people were over-represented among severe cases compared with previous seasonal outbreaks.^4,142,143

In 2017, a severe influenza season with the highest levels of activity since the 2009 pandemic year was recorded. New South Wales hospitals introduced rapid influenza testing, which may have partly contributed to the increased number of laboratory-confirmed notifications of influenza. Nationally, there were about 750 deaths among notified cases of laboratory-confirmed influenza in 2017,¹⁴⁴ while Australian modelling has demonstrated that at least twice the average number of deaths occurred in 2017 compared to the average for 2010–2018.¹³⁹

During the COVID-19 pandemic there was reduced circulation of influenza virus and lower levels of influenza vaccine coverage compared to previous years. COVID-19 related public health measures and the community’s adherence to public health messages have also likely had an effect on transmission of acute respiratory infections, including influenza.¹⁴⁵ A resurgence of influenza activity was observed in 2022 with early season circulation.¹⁴⁶ Ongoing seasonal circulation of influenza, as seen in 2023–24, is expected to continue emphasising that vulnerable groups are at high risk of influenza complications if they are not vaccinated.

Each year, the World Health Organization recommends the strains to be included in influenza vaccines based on global influenza epidemiology.^147,148 The Australian Influenza Vaccine Committee uses this recommendation to determine the influenza virus composition of vaccines for use in Australia.¹⁴⁹  

Both trivalent and quadrivalent vaccines have been used in Australia. Currently all vaccines are trivalent. The effectiveness against the shared strains of trivalent and quadrivalent influenza vaccines are generally equivalent across all age groups.^150-154

Efficacy and effectiveness of influenza vaccines

The efficacy and effectiveness of influenza vaccines of similar composition depend on the:

• age and immunocompetence of the vaccine recipient
• degree of similarity between the virus strains in the vaccine and those circulating in the community^44,155-162

LAIV

LAIV is a live vaccine that is administered intranasally, providing an alternative to the intramuscularly administered IIV. Evidence from overseas, particularly the United States, Canada and the United Kingdom where LAIV has been in use for many years, indicates high acceptability of intranasally-administered influenza vaccine.^163,164The effectiveness of LAIV appears comparable to inactivated influenza vaccines.^84,165-168 Clinical trials show that the vaccine efficacy of LAIV against influenza infection is equivalent to that of IIV (OR=0.81; 95%CI: 0.49–1.34).⁸⁴ LAIV has been used in a school-based influenza vaccination program in the United Kingdom since 2013, with multiple studies demonstrating reductions in influenza disease. Data across three seasons showed that vaccine effectiveness against influenza hospitalisations was 50.1% (95%CI: 31.2–63.8), while more recent data from the 2023/24 season showed LAIV reduced confirmed influenza presentations to primary care by 65% (95%CI: 41–79).^169,170 Data from the UK also shows that when LAIV vaccination coverage is high among children, there are indirect benefits to adults who were not targeted for vaccination.¹⁷¹

Influenza vaccines in young children

Influenza vaccine is registered for use from 6 months of age at which time young children can be vaccinated. Because young children are immunologically naïve to influenza, they need 2 doses of influenza vaccine when immunised for the first time. This maximises the immune response to all vaccine strains.^172,173 Evidence from a systematic review suggests that while vaccine effectiveness improved with 2 doses in very young children, there is little to no additional benefit in vaccinating children aged ≥3 years with an additional dose of influenza vaccine.⁵⁴

Young children gain similar levels of protection as older children and adults. Vaccine effectiveness is approximately 65% against laboratory-confirmed influenza in children aged 6–59 months in a season when the vaccine and circulating strains are well matched.^174,175-180 

Influenza vaccines in adults aged ≥50 years

Currently in Australia, the adjuvanted influenza vaccine is registered for adults aged 50 years and above and high dose influenza vaccine is registered for use in adults aged 60 years and above.

Standard-dose influenza vaccines were shown to provide less protection against influenza in people aged ≥65 years than in younger people. Because of this, ‘enhanced’ formulations were developed to increase the immune system’s response to the vaccine. These enhanced vaccines increase protection compared with standard-dose vaccines, especially against influenza A (H3N2), which is more common and severe in older people.^98,181,182

In large post-licensure studies of community-dwelling adults aged ≥65 years, adjuvanted influenza vaccine was between 4.7% and 33% more effective in preventing hospitalisation from influenza or pneumonia than standard-dose influenza vaccine.^183-186

Two post-licensure studies in adults aged ≥65 years found that the high-dose influenza vaccine was between 23% and 47% more effective at preventing influenza or penumonia associated mortality than standard-dose influenza vaccines.^187,188 High-dose influenza vaccine was also found to be between 2.0% and 27.0% more effective in preventing hospitalisation from influenza or pneumonia than standard-dose influenza vaccines in large post-licensure studies of adults aged ≥65 years.^{182,186,189-197} A meta-analysis of 15 studies confirms this greater protection against a range of influenza disease related outcomes.¹⁹⁸

Large post-licensure studies comparing high-dose influenza vaccine and adjuvanted influenza vaccine in adults aged ≥65 years, show little to no difference between these two vaccines in effectiveness aginst a range of influenza outcomes (relative vaccine effectiveness ranged from -3.0% and 7.7%).^182,186

Influenza vaccines in pregnant women

In pregnant women, standard-dose egg-based inactivated influenza vaccine is:^199,200

• approximately 50% effective in reducing laboratory-confirmed influenza
• 65% effective against inpatient hospital admissions for acute respiratory illness

Vaccinating pregnant women against influenza also protects their infants against laboratory-confirmed influenza. This is due to transplacental transfer of high-titre influenza-specific antibodies. A recent systematic review concluded that maternal influenza vaccination reduces laboratory-confirmed influenza in infants <6 months of age by about 48%.²⁰¹ The vaccine protects infants for up to 6 months after birth.

Inactivated influenza vaccines should be given in pregnancy instead of live attenuated influenza vaccines (LAIV). However, there is no evidence of risk associated with LAIV in pregnancy. 

Cell-based compared with egg-based vaccines

The cell-based influenza vaccine, Flucelvax, is prepared in Madin-Darby canine kidney (MDCK) cells. The virus is inactivated, disrupted and purified through several steps. Eggs are not used in the manufacturing process. Antigenic mismatch may be less likely to occur for cell-based influenza vaccines than for egg-based influenza vaccines. Repeated passages in eggs can result in an antigenic mismatch with the circulating influenza strains. This issue has been particularly problematic with some strains of A/H3N2 virus, which requires many passages in eggs to achieve high virus titres.^202,203

Studies have generally shown that cell-based influenza vaccine has similar effectiveness against laboratory-confirmed influenza to standard-dose egg-based influenza vaccine. It has been noted, for some influenza seasons, that cell-based vaccines had a small observed increased benefit in protection against influenza-associated hospitalisations.^181,203-208 This small benefit has been variable and the studies have the limitations associated with observational studies.

Duration of immunity

Protection against influenza requires annual vaccination with a vaccine containing the most recent strains. Recent evidence suggests that protection after influenza vaccination may begin to wane after 3–4 months.^{172,173,178,205-208}Low levels of protection may last another year for some strains, if the prevalent circulating strain remains the same or if there is only minor antigenic drift.^44,162

It is not clear whether repeated annual vaccination affects year-to-year vaccine effectiveness. An Australian study reported sustained or increased vaccine effectiveness against influenza A and B illness and hospitalisation, particularly among children aged 2–8 years who received influenza vaccine in previous seasons.²⁰⁴ Also, data collected over 6 influenza seasons show that repeated annual influenza vaccination among older people who live in the community is associated with a 15% reduction in the risk of annual mortality compared with first-time vaccination.²⁰⁹

However, a few studies, mainly in the United States, suggest that influenza vaccines are not as effective if they are repeated year after year.^210,211 Reduced effectiveness following repeated annual vaccination has not been observed consistently across studies. Better understanding of these effects is needed to determine clinical significance and guide recommendations.

Despite conflicting opinion about the impact of repeated annual vaccination on vaccine effectiveness, vaccinated people are still better protected against influenza than unvaccinated people.

Transport according to National Vaccine Storage Guidelines: Strive for 5.²¹² Store at +2°C to +8°C. Do not freeze. Protect from light.

Discard influenza vaccines appropriately when they reach their expiry date. This is to avoid accidentally using a product with the incorrect formulation the following year.

Laboratory-confirmed cases of influenza are notifiable in all states and territories in Australia.

The Communicable Diseases Network Australia national guidelines for influenza infection²¹³ have details about the management of influenza cases and contacts.

State and territory public health authorities can provide further advice about the public health management of influenza, including in residential care facilities.

Egg allergy

The product information for Flumist (LAIV) lists severe allergy (e.g. anaphylaxis) to egg as a contraindication. The product information for Fluzone lists a known systemic hypersensitivity reaction, such as anaphylaxis, to any component of the vaccine as a contraindication.

The Australian Technical Advisory Group on Immunisation recommends that all people with egg allergies, including those with a history of anaphylaxis, can receive an age-appropriate influenza vaccine. See Precautions.

Dosage in children

The product information for influenza vaccines registered for use in children state that children <9 years of age who have not previously received an influenza vaccine should be given 2 doses of vaccine, 4 weeks apart.

The Australian Technical Advisory Group on Immunisation recommends that children aged <2 years who have not previously received influenza vaccine should be given 2 doses of vaccines, 4 weeks apart. Children aged ≥2 years who do not have a medical at-risk condition should receive 1 dose of vaccine, regardless of whether they have previously been vaccinated against influenza. See Doses for children <2 years of age.

Children aged <9 years with a medical risk conditions receiving influenza vaccine for the first time should receive 2 doses of vaccine, 4 weeks apart.

Fluad

The product information for Fluad states that Fluad should be given intramuscularly.

The Australian Technical Advisory Group on Immunisation recommends that Fluad can also be given subcutaneously.

Fluzone

The product information for Fluzone states that Fluzone should be given intramuscularly.

The Australian Technical Advisory Group on Immunisation recommends that Fluzone can also be given subcutaneously.

Flucelvax

The product information for Flucelvax states that Flucelvax should be given intramuscularly.

The Australian Technical Advisory Group on Immunisation recommends that Flucelvax can also be given subcutaneously.

Fluzone High-Dose

The product information for Fluzone High-Dose states that Fluzone High-Dose should be given intramuscularly.

The Australian Technical Advisory Group on Immunisation recommends that Fluzone High-Dose can also be given subcutaneously.

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