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 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 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)

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

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

• People aged 6 months to <65 years should receive quadrivalent influenza vaccine (QIV).
• People aged ≥65 years should receive adjuvanted quadrivalent influenza vaccine (QIV), but may receive a standard QIV if the adjuvanted QIV 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

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.

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 health.gov.au website 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 vigorously before injection. Influenza vaccines are preferably given by intramuscular injection. However, they may also be given subcutaneously. 

Doses for children <9 years of age

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

Children who received 1 or more doses of trivalent or quadrivalent influenza vaccine in a previous season only need 1 dose of influenza vaccine in the current and future seasons. This is regardless of whether a trivalent or quadrivalent vaccine is used.^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 quadrivalent influenza vaccine in the same season is not recommended.

Similarly, if they have received quadrivalent 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 inactivated influenza vaccines at any time before or after, or with, any other vaccine. However, when children receive 13vPCV and inactivated influenza vaccine at the same time, there is one report of a small increased risk of fever and febrile convulsions, compared with receiving the vaccines separately. This study found that the risk was 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.⁵¹

However, immunisation providers should:

• advise parents and carers of the possible risk
• provide the option of administering these 2 vaccines on separate days, with an interval of at least 3 days

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.

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 trivalent influenza vaccine (TIV) intramuscularly. These include:

• induration
• swelling
• redness
• pain^66-68

Studies directly comparing inactivated trivalent and quadrivalent formulations in children and adults showed that the safety profiles were similar.^69-72

Systemic adverse events

1–10% of people who receive standard inactivated TIVs will develop:

• fever
• malaise
• myalgia^66-68,73-75

These adverse events may start a few hours after vaccination and may last for 1–2 days.^66,67,73

Immediate adverse events following influenza vaccination are very rare. They can include hives, angioedema or anaphylaxis.^52,57

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.^52,57 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 influenza vaccines. 

Around 30% of people who received Fluad TIV reported injection site reactions, compared with around 20% of people who received standard TIV. Both groups reported similar rates (about 30%) of systemic reactions.⁷⁶  Overall, a similar proportion of people vaccinated with Fluad Quad (aQIV) experience injection site and systemic reactions as those vaccinated with Fluad TIV.⁷⁷

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.^76-79

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.^80-82

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.⁸³ 

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.^84-87 Several systematic reviews have shown no association between influenza vaccination in pregnancy and adverse birth outcomes.^88,89

Guillain–Barré syndrome 

In 1976, a small increased risk of Guillain–Barré syndrome (GBS) was associated with 1 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.⁹⁰ 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 in Precautions, 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.^91-93 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.^94,95

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: 

• higher fevers
• febrile convulsions
• otitis media
• gastrointestinal symptoms¹⁰¹

Infection in young infants may be associated with more non-specific symptoms.^101,102

Complications of influenza

Complications of influenza include:

• acute bronchitis
• croup
• 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.

Secondary bacterial pneumonia is more common than primary viral pneumonia.

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.^103,104 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.^96,105-107

Each year, the World Health Organization recommends the strains to be included in influenza vaccines based on global influenza epidemiology.¹¹⁸ The Australian Influenza Vaccine Committee uses this recommendation to determine the influenza virus composition of vaccines for use in Australia.¹¹⁹

Influenza vaccines 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.

Although these 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.

Inactivated QIVs contain 4 influenza virus strains – 2 influenza A subtypes and 2 influenza B lineages. QIVs have been registered for use in Australia since 2014, and have been in widespread use since 2016.

Standard influenza vaccines contain 15 µg of haemagglutinin per strain per dose with no adjuvant. In 2020 an adjuvanted QIV (Fluad Quad) is available. The adjuvanted QIV contains MF59 as the adjuvant, and the standard 15 µg of haemagglutinin per strain per dose. The vaccine is formulated to induce a greater immune response than standard influenza vaccines.

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^44,120-127 

QIVs and TIVs

Most evidence regarding the efficacy of QIVs was established on the basis of non-inferior immune responses compared with TIVs against the 3 strains common to both vaccines.

A systematic review estimated the efficacy of standard TIV to be 59% against laboratory-confirmed influenza in healthy adults <65 years old; this varies with influenza season.¹²⁸ Based on studies comparing the immune response of QIVs and TIVs, the efficacy of QIVs is expected to be similar. In one season in the United States when the circulating and vaccine strains were well matched, QIVs were about 54% effective against laboratory-confirmed influenza.¹²⁹

QIVs protect against an extra influenza B lineage compared with TIVs. The benefit of this cannot be predicted for any influenza season, because it depends on factors such as:

• the proportion of circulating influenza viruses that is attributable to the influenza B lineage that is not in the TIV, which varies every year⁹⁷
• antigenic mismatch between vaccine and circulating strains
• cross-protection against non-vaccine B strains afforded by the strain in the TIV
• a person’s pre-existing immunity to the circulating strains of influenza 

Influenza vaccines in young children

Young children can receive influenza vaccine from 6 months of age. 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.^130,131 Vaccination against the additional influenza B lineage in QIVs is expected to be particularly beneficial for children, who are more likely to be immunologically naive to all strains of influenza.¹³²

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.^133-138

Influenza vaccines in adults aged ≥65 years

Standard-dose influenza vaccines 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.^78,139

Most evidence on the effectiveness of adjuvanted influenza vaccine compares adjuvanted TIV with standard dose TIV. There are no studies on adjuvanted QIV effectiveness, but a study comparing adjuvanted QIV with adjuvanted TIV showed that the adjuvanted QIV not inferior in terms of generating immune responses against the shared strains and is superior for the B strain not included in the adjuvanted TIV.⁷⁷ In a large post-licensure study of community-dwelling adults aged ≥65 years, the adjuvanted TIV (Fluad) was about 25% more effective in preventing hospitalisation from influenza or pneumonia than standard TIV.¹⁴⁰

Influenza vaccines in pregnant women

In pregnant women, standard TIV is: 

• approximately 50% effective in reducing laboratory-confirmed influenza 
• 65% effective against inpatient hospital admissions for acute respiratory illness^141,142

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%.¹⁴³ The vaccine protects infants for up to 6 months after birth. 

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.^{130,131,136,144-147} 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.^44,127 

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.¹⁴⁸ 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.¹⁴⁹ 

However, a few studies, mainly in the United States, suggest that influenza vaccines are not as effective if they are repeated year after year.^150,151 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.

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

The Communicable Diseases Network Australia national guidelines for influenza infection¹⁵³ 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.

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