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
• homeless people
• 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
• people who provide essential community services
• people who are travelling during influenza season

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

• immunocompromising conditions, such as HIV, malignancy, functional or anatomical asplenia, and chronic steroid use
• receiving immuno-oncology therapy
• received a haematopoietic stem cell or solid organ transplant
• cardiac disease
• Down syndrome
• obesity
• chronic respiratory conditions
• chronic neurologic conditions
• chronic liver disease
• other chronic illnesses that need medical follow-up or hospitalisation
• long-term aspirin therapy in children (aged 6 months to 10 years)
• preterm infants (<37 weeks gestation)

Also see People at risk of severe disease from influenza.

How

People are recommended to receive influenza vaccine every year.

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

• children aged 6 months to <9 years 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 <65 years should receive a standard-dose egg-based or cell-based influenza vaccine.
• People aged ≥65 years should receive adjuvanted (NIP-funded) or high-dose influenza vaccine, but may receive standard-dose egg-based or cell-based 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.

See Infographic. Vaccination for healthy ageing.

Influenza vaccines available in Australia

All the 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 these vaccines are quadrivalent as they contain 4 influenza virus strains – 2 influenza A subtypes and 2 influenza B lineages.

Standard-dose egg-based 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.

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

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

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

Doses for children <9 years of age

Children aged 6 months to <9 years receiving influenza vaccine for the first time are recommended 2 doses at least 4 weeks apart. This maximises the immune response to the vaccine strains. 

Children who 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. ^49,50

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

Doses for children ≥9 years and adults

People aged ≥9 years need 1 dose of influenza vaccine every year, regardless of whether they have ever had influenza vaccine before.

For adults aged ≥65 years who have already received an adjuvanted influenza vaccine in the current influenza season, an extra dose of any influenza vaccine in the same season is not recommended.

Similarly, if they have received a standard-dose influenza vaccine, an extra dose of adjuvanted influenza vaccine in the same season is not recommended.

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

Co-administration with other vaccines

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

For more information refer to the COVID-19 handbook chapter.

The safety of concomitant administration of the adjuvanted vaccines Fluad Quad and Shingrix has not been studied. It is acceptable to co-administer these vaccines on the same day if necessary. However, given the lack of data on co-administration of these adjuvanted vaccines, it is currently considered preferable to separate their administration by a few days.

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).^51-53  This lower immunogenicity was not seen in co-administration studies with unadjuvanted quadrivalent influenza vaccine.^51,53,54 The clinical significance of the lower immunogenicity is not known.

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.^51,53 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.

There is a possible small increased risk of fever in children if 13vPCV is given at the same time as influenza vaccine. See Contraindications and precautions and Pneumococcal disease.

Interchangeability of influenza vaccines

Different age-appropriate brands are interchangeable for people who need 2 doses of influenza vaccine in a single season (see Table. Recommended doses of influenza vaccine by age group).

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 <9 years 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.

COVID-19 vaccines can be co-administered (i.e. on the same day) with an influenza vaccine. 

If a person had a 2023 influenza vaccine in late 2023 or early 2024, they are still recommended to receive a 2024 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

See Precautions for more details about influenza vaccination for people with a known egg allergy.

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  and cell-based 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.^55-60

A 2012 review of published studies included 4172 egg-allergic patients. 513 of these patients reported a history of severe allergic reaction to egg. The review found no cases of anaphylaxis after receiving an inactivated influenza vaccine.⁵⁹

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.⁶¹ 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. 

People with latex allergy

Although the product information for Fluarix Tetra states that some preparations of the vaccine cannot be considered latex-free, these preparations are not supplied in Australia.

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.^62-64

Influenza vaccination is generally not recommended for people with a history of GBS whose first episode occurred within 6 weeks of receiving an influenza vaccine. There is limited data on the risk of recurrence of GBS in people where the first episode occurred within 6 weeks of influenza vaccination (i.e. the first episode was possibly triggered by the vaccine). In these people, discuss the potential for recurrence if vaccinated, the potential for exacerbation following influenza infection, and other protective strategies (e.g. vaccination of household members). Vaccination can be considered in special circumstances, such as when an alternative cause for GBS, such as Campylobacter jejuni infection, was found or the risk of influenza disease is considered high.

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,^65,66 but a more recent study on patients receiving treatment with a single checkpoint inhibitor did not.^67-70

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 more than one of these treatments.

Children who need both influenza vaccine and 13vPCV

Children can receive 13vPCV and influenza vaccine at the same visit if they need both vaccines.

One study found a slightly higher risk of fever and febrile convulsions in children aged 6 months to <5 years (especially those aged 12–24 months) when they received influenza vaccine and 13vPCV at the same time, compared with receiving the vaccines separately. ⁷¹ The risk of febrile convulsions as about 18 more cases per 100,000 doses in children aged 6 months to <5 years. The highest risk was 45 per 100,000 doses in children aged 16 months. This is a relatively small risk increase. 

A later study did not show an association between co-administering these 2 vaccines and febrile seizures.⁷² 

Immunisation providers should advise parents and carers of the possible risk.

See Pneumococcal disease.

Post-vaccination symptoms may mimic influenza infection. None of the influenza vaccines available in Australia contain live influenza viruses, so they cannot cause influenza.

Injection site reactions

Injection site reactions occur in more than 10% of people who receive standard-dose influenza vaccine intramuscularly. These include:^73-75

• induration
• swelling
• redness
• pain

Studies directly comparing inactivated trivalent and quadrivalent formulations in children and adults showed that the safety profiles were similar.^76-79

Systemic adverse events

1–10% of people who receive standard-dose influenza vaccines will develop:^{73-75, 80-82}

• fever
• malaise
• myalgia

These adverse events may start a few hours after vaccination and may last for 1–2 days.^73,74,80

Immediate adverse events following influenza vaccination are very rare. They can include hives, angioedema or anaphylaxis.^55,60

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.^55,60 See People with egg allergy in Contraindications and precautions.

Adverse events after adjuvanted influenza vaccine

Clinical trials show a higher rate of injection site reactions in adults aged ≥65 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.⁸³ Overall, a similar proportion of people vaccinated with Fluad Quad experience injection site and systemic reactions as those vaccinated with Fluad.⁸⁴

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.^83-86

Adjuvanted influenza vaccine is only registered for use in people aged ≥65 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 people aged ≥65 years.^87-89

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. 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.⁹⁰ 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.⁹¹ 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.⁹² 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.⁹³

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

Influenza vaccine is safe to give during any stage of pregnancy or while breastfeeding for both the mother and her baby.^94-97 Several systematic reviews have shown no association between influenza vaccination in pregnancy and adverse birth outomes.^98,99

Standard-dose egg-based influenza vaccines are currently preferred for use in pregnancy because a large body of evidence supports their safety in pregnant women. While the use of cell-based influenza vaccines in pregnancy has not been assessed, there are no theoretical concerns regarding their safety in pregnant women.

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.¹⁰⁰ The risk of GBS following influenza illness is estimated to be much higher to about 15 times the risk of GBS following influenza vaccine.^101,102 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.^103-105 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.^106,107

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.¹⁰⁸

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.^109,110 However, B/Yamagata strain has not been detected in global circulation since 2020.¹¹¹

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.¹⁰⁸Antigenic 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.^112,113

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.¹⁰⁸

Transmission

People transmit influenza from person to person:

• through virus-containing respiratory aerosols produced during coughing or sneezing
• by direct contact with respiratory secretions¹⁰⁸

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.¹⁰⁸

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: ¹¹⁴

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

Infection in young infants may be associated with more non-specific symptoms.^114,115

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.^116,117 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.^108,118-120

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.¹¹⁴ Changes in influenza detection methods have affected influenza detection and notification patterns.¹²¹ 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.^5,23,122,123

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.¹²⁴ 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 100,000.¹²⁴ 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.¹²⁴

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.¹²⁵ In Australia, as in other developed countries, older people and children <5 years of age have the highest rates of influenza hospitalisation.^{4,121,126,127} 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.^4,128,129

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 2017,¹³⁰ while Australian modelling has demonstrated that at least twice the average number of deaths occurred in 2017 compared to the average for 2010–2018.¹²⁵

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.¹³¹ As the international borders reopened from November 2021 and greater population movement occurred, a resurgence of influenza activity was observed in 2022 with early season circulation.¹³² Ongoing seasonal circulation of influenza, as seen in 2023, 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.^133,134 The Australian Influenza Vaccine Committee uses this recommendation to determine the influenza virus composition of vaccines for use in Australia.¹³⁵

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^44,136-143

Quadrivalent influenza vaccines and trivalent influenza vaccines

Most evidence regarding the efficacy of quadrivalent influenza vaccines was established on the basis of non-inferior immune responses against the 3 strains in common with trivalent influenza vaccines.

A systematic review estimated the efficacy of trivalent standard-dose egg-based influenza vaccine to be 59% against laboratory-confirmed influenza in healthy adults <65 years old; this varies with influenza season.¹⁴⁴ Based on studies comparing the immune response of quadrivalent and trivalent standard-dose egg-based influenza vaccines, the overall efficacy of the quadrivalent vaccines is expected to be similar to the trivalent vaccines, noting that quadrivalent vaccines protect against an extra influenza B lineage. In one season in the United States when the circulating and vaccine strains were well matched, quadrivalent standard-dose egg-based influenza vaccines were about 54% effective against laboratory-confirmed influenza.¹⁴⁵

The World Health Organization (WHO) and the Australian Influenza Vaccine Committee (AIVC) have recommended that the B Yamagata lineage component is no longer warranted in seasonal influenza vaccines, as this lineage has not been seen for several years. However, the quadrivalent vaccines available in Australia continue to include the B Yamagata component, and ATAGI supports the use of these vaccines in 2024.

Influenza vaccines in young children

The influenza vaccine is registered for use from 6 months of age at which time young children can be vaccinated. Because these children are immunologically naive to influenza, they need 2 doses of influenza vaccine when immunised for the first time. This maximises the immune response to all vaccine strains.^146,147

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.^148,149-154 

Influenza vaccines in adults aged ≥60 years

It is important to note that adjuvanted influenza vaccine is not registered for 60 to 64 years. High dose influenza vaccine is the only enhanced influenza vaccine registered for use in adults 60-64 years of age.

Standard-dose egg-based 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 egg based and cell-based vaccines, especially against influenza A (H3N2), which is more common and severe in older people.^85,155,156

Most evidence on the effectiveness of adjuvanted influenza vaccine compares trivalent adjuvanted vaccine with trivalent or quadrivalent standard-dose egg-based influenza vaccines. There are no studies directly comparing quadrivalent adjuvanted and standard-dose egg-based influenza vaccines. However, a study comparing quadrivalent and trivalent adjuvanted influenza vaccines showed that the quadrivalent vaccine was not inferior in terms of generating immune responses against the shared strains, and was superior for the B strain not included in the trivalent vaccine.⁸⁴ 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 influenza vaccine.^157-160

Most evidence on the effectiveness of high-dose influenza vaccines compares trivalent high dose vaccine with trivalent or quadrivalent standard-dose egg-based influenza vaccines and with trivalent adjuvanted influenza vaccine. There are no studies measuring effectiveness outcomes that directly compare quadrivalent high-dose vaccine and standard-dose egg-based influenza vaccines. However, a study comparing quadrivalent and trivalent high-dose influenza vaccines showed that the quadrivalent vaccine was not inferior in terms of generating immune response against the shared strains, and was superior for both B strains not included in the trivalent vaccine.¹⁶¹

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 egg-based influenza vaccines.^162,163 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 egg-based influenza vaccines in large post-licensure studies of adults aged ≥65 years.^156,164-173 A meta-analysis of 15 studies confirms this greater protection against a range of influenza disease related outcomes.¹⁷⁴

There are no studies that directly compare quadrivalent high-dose influenza vaccine and quadrivalent adjuvanted influenza vaccine. Large post-licensure studies comparing trivalent high-dose influenza vaccine and trivalent 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%).^156,168

Influenza vaccines in pregnant women

In pregnant women, standard-dose egg-based influenza vaccine is:^175,176

• 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%.¹⁷⁷ The vaccine protects infants for up to 6 months after birth.

Cell-based compared with egg-based vaccines

The cell-based influenza vaccine, Flucelvax Quad, 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.^178,179

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.^156,180-185 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.^{146,147,152,186-189} 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.^44,143

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.¹⁹⁰ 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.¹⁹¹

However, a few studies, mainly in the United States, suggest that influenza vaccines are not as effective if they are repeated year after year.^192,193 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 5.¹⁹⁴ 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 infection¹⁹⁵ 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 Influvac Tetra lists allergy to egg as a contraindication.

The product information for Afluria Quad state that people with egg allergy (non-anaphylaxis) can receive an age-appropriate dose.

The Australian Technical Advisory Group on Immunisation recommends that all people with egg allergies can receive an age-appropriate influenza vaccine. See Precautions.

Fluad Quad

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

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

FluQuadri

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

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

Flucelvax Quad

The product information for Flucelvax Quad states that Flucelvax Quad should be given intramuscularly. The Australian Technical Advisory Group on Immunisation recommends that Flucelvax Quad 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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