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 a standard influenza vaccine.
• People aged ≥65 years should receive adjuvanted influenza vaccine, but may receive a standard influenza vaccine if the adjuvanted 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 (standard influenza vaccines 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 (egg-based) influenza vaccines contain 15 µg of haemagglutinin per strain per dose with no adjuvant. Similarly, the available cell-based influenza vaccine contains 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 influenza vaccines.

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 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 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 standard influenza vaccine in the same season is not recommended.

Similarly, if they have received a standard 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 any time before or after, or with, most other vaccines.

Safety and immunogenicity data on coadministration of influenza vaccine and any COVID-19 vaccines are not yet available. Receiving an influenza vaccine and a COVID-19 vaccine on the same day is not recommended.

A minimum interval of 14 days between an influenza vaccine and a COVID-19 vaccine is preferred. Influenza vaccine can be given before or after any dose of a COVID-19 vaccine as long as a minimum interval of 14 days is observed. People in phase 1a of the COVID-19 vaccination program should receive the COVID-19 vaccine as soon as it is available to them, and then receive their influenza vaccine at least 14 days later. People in later phases of the COVID-19 vaccination program should receive their influenza vaccine as soon as it is available, and then receive their COVID-19 vaccine when it becomes available to them, at least 14 days later. For more information refer to ATAGI's advice on the Relative Timing of Administering Influenza and COVID-19 Vaccines in 2021.

The adjuvanted influenza vaccine, Fluad Quad, should not be co-administered with the adjuvanted subunit zoster vaccine, Shingrix (which is anticipated to be available in Australia from mid 2021). The safety of concomitant administration of two adjuvanted vaccines has not been studied.

See also 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 influenza vaccine intramuscularly. These include:^66-68

• induration
• swelling
• redness
• pain

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 influenza vaccines will develop:^66-68,73-75

• fever
• malaise
• myalgia

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.^51,56

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.^51,56 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 reported injection site reactions, compared with around 20% of people who received standard influenza vaccine. Both groups reported similar rates (about 30%) of systemic reactions.⁷⁶ Overall, a similar proportion of people vaccinated with Fluad Quad experience injection site and systemic reactions as those vaccinated with Fluad.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

Adverse events after cell-based influenza vaccine

Cell-based influenza vaccines have a similar safety profile to standard influenza vaccines. In one 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 influenza vaccine. Systemic reactions were reported by 37% and 30%, respectively.⁸³ 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 influenza vaccine. Systemic reactions were reported by 25% and 23%, respectively.⁸⁴ 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.⁸⁵

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.^86-89 Several systematic reviews have shown no association between influenza vaccination in pregnancy and adverse birth outomes.^90,91

Standard 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 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.⁹²If 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.^93-95 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.^96,97

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: ¹⁰³

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

Infection in young infants may be associated with more non-specific symptoms.^103,104

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.^105,106 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.^98,107-109

Each year, the World Health Organization recommends the strains to be included in influenza vaccines based on global influenza epidemiology.¹²⁰ The Australian Influenza Vaccine Committee uses this recommendation to determine the influenza virus composition of vaccines for use in Australia.¹²¹

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^44,122-129

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 influenza vaccine to be 59% against laboratory-confirmed influenza in healthy adults <65 years old; this varies with influenza season.¹³⁰ Based on studies comparing the immune response of quadrivalent and trivalent standard 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 influenza vaccines were about 54% effective against laboratory-confirmed influenza.¹³¹

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.^132,133

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.^134-139 

Influenza vaccines in adults aged ≥65 years

Standard 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 vaccines, especially against influenza A (H3N2), which is more common and severe in older people.^78,140

Most evidence on the effectiveness of adjuvanted influenza vaccine compares trivalent adjuvanted vaccine with trivalent or quadrivalent standard influenza vaccines. There are no studies directly comparing quadrivalent adjuvanted and standard 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.⁷⁷ 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.^141-144

Influenza vaccines in pregnant women

In pregnant women, standard influenza vaccine is:^145,146

• 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%.¹⁴⁷ 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 standard influenza vaccines. Repeated passages in eggs can result in an antigenic mismatch with the circulating influenza strains. This issue has been particularly problematic for the recent A(H3N2) virus, which requires many passages in eggs to achieve high virus titres.^148,149

Studies have shown that cell-based influenza vaccine has similar efficacy against laboratory-confirmed influenza to standard influenza vaccine.^150,151

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.^{132,133,137,152-155} 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.^44,129

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.¹⁵⁶ 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.¹⁵⁷

However, a few studies, mainly in the United States, suggest that influenza vaccines are not as effective if they are repeated year after year.^158,159 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 infection¹⁶¹ 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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