Japanese encephalitis

What

Japanese encephalitis (JE) is caused by infection with the mosquito-borne JE virus. The disease mainly affects the central nervous system.

Who

JE vaccines are recommended for:

• routine vaccination of laboratory workers who may be exposed to JE virus
• routine vaccination of travellers spending 1 month or more in endemic areas during the JE virus transmission season
• routine vaccination of people who live or work on the outer islands of Torres Strait

How

JE vaccination is recommended for all research laboratory workers who may be exposed to the virus. These people may need a booster dose if they have ongoing risk of exposure to the virus.

JE vaccination is recommended for travellers spending 1 month or more in endemic areas in Asia and Papua New Guinea during the JE virus transmission season.

JE vaccination is recommended for residents of the outer islands in Torres Strait and non-residents who will be living or working on the outer islands of Torres Strait for 30 days or more during the wet season.

People who are at ongoing risk of acquiring JE may need booster doses.

Why

JE is a significant public health problem in many parts of Asia, and occasional outbreaks have occurred in Torres Strait. JE has a high case-fatality rate of around 30%. Around 50% of people who survive the acute illness will have neurological sequelae.

Japanese encephalitis 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.

Dose and route

Recommended doses and schedules are shown in Table. Recommended doses of Japanese encephalitis vaccines.

Imojev

People aged ≥9 months can receive Imojev. The dose is 0.5 mL given by subcutaneous injection. For dose sparing purposes, Imojev may be given intradermally in special circumstances such as in the context of outbreaks, expanded eligibility in risk areas, and ongoing supply considerations.⁵

JEspect

JEspect is given by intramuscular injection. The primary dose needed depends on the age of the person:

• infants and children aged ≥2 months to <3 years should receive 2 doses, each of 0.25 mL, 28 days apart. The vaccine comes as a pre-filled syringe and contains 0.5mL. Immunisers should draw up 0.25mL and discard the remaining half dose
• children aged ≥3 years and adults should receive 2 doses, each of 0.5 mL, 28 days apart

Co-administration with other vaccines

Imojev

People can receive Imojev at the same time as:

• yellow fever vaccine⁷
• MMR (measles-mumps-rubella) vaccine⁸

Use separate syringes and inject in separate limbs. If a person does not receive Imojev and yellow fever vaccine (or other live vaccines) at the same time, they should receive them at least 4 weeks apart.

No data on co-administration with other vaccines are available.

JEspect

People can receive JEspect at the same time as:

• hepatitis A vaccine⁹
• quadrivalent (ACWY) meningococcal conjugate vaccine¹⁰
• rabies vaccine¹⁰

Use separate syringes and inject in separate limbs.

Co-administration with other vaccines (including yellow fever vaccine) has not been assessed.

Interchangeability of JE vaccines

Try to use the same vaccine for the booster dose as was used for the primary course. No studies have looked at the immune response to an Imojev booster in people who received a primary course of JEspect, or vice versa.

However, both vaccines use the same virus strain, so a booster using the alternative vaccine should give a satisfactory immune response, based on first principles.

Some people would have been previously vaccinated with the mouse brain-derived Japanese encephalitis (JE) vaccine, JE-Vax. These people can receive either Imojev or JEspect if they have an ongoing risk of JE virus exposure. ^11-13

Contraindications

Japanese encephalitis (JE) vaccines are contraindicated in people who have had:

• anaphylaxis after a previous dose of any JE vaccine
• anaphylaxis after any component of a JE vaccine

Imojev is contraindicated in people who are immunocompromised and in pregnant women because it is a live attenuated viral vaccine. Women should avoid pregnancy for 28 days after vaccination.

Breastfeeding women should not receive Imojev because it is not known whether the virus is excreted in breast milk.

See Table. Vaccines that are contraindicated in pregnancy: live attenuated vaccines in Vaccination for women who are planning pregnancy, pregnant or breastfeeding for more details.

Precautions

People with an acute febrile illness should not receive JE vaccines.

People who are immunocompromised

JEspect is an inactivated vaccine, so it is not expected to cause any safety concerns in people who are immunocompromised. However, there are few data on the safety and efficacy of JEspect in those who are immunocompromised. These people may not mount an adequate immune response to the vaccine.

Women who are pregnant or breastfeeding

JE vaccine is not routinely recommended for pregnant or breastfeeding women. However, pregnant women at risk of acquiring JE are recommended to receive JEspect. JE virus infection during the 1st and 2nd trimesters has been associated with miscarriage. No adverse outcomes of pregnancy have been attributed to vaccination with JEspect.

Breastfeeding women who are at increased risk of acquiring JE are recommended to receive JEspect. However, no specific data are available about using JEspect in breastfeeding women.

Vaccination after immunoglobulin or blood product administration

Do not give Imojev within 6 weeks after giving immunoglobulins or immunoglobulin-containing blood products. It is preferable to wait 3 months.

People with possible IFNAR1 deficiency

IFNAR1 deficiency is a rare inherited condition affecting some people of Western Polynesian heritage, including Tongan, Samoan and Niuean. It is associated with severe illness and death from certain viral infections and also potentially from some live attenuated viral vaccines, including the measles, mumps and rubella (MMR) vaccine, the yellow fever virus vaccine and potentially the live attenuated Japanese encephalitis vaccine, though disseminated Japanese encephalitis post vaccination has not been seen.^13,14 Currently, diagnosis of IFNAR1 deficiency prior to vaccination is challenging and there is no established treatment.

Healthcare providers need to be aware that people of Western Polynesian heritage who present for medical attention and are very unwell in the 1-2 weeks following a live Japanese encephalitis vaccine may need further investigation by an immunologist to assess for an immune deficiency. Any suspected adverse event following immunisation should be reported. Family members of individuals who have had a severe reaction to a live attenuated viral vaccine, or are related to someone with known IFNAR1 deficiency, should be referred to an immunologist before having these vaccine.

Advice on vaccination of children or adults who have been diagnosed with IFNAR1 or any other specific immune deficiency should be sought from their immunologist. In such individuals the inactivated JEspect is recommended, noting this vaccine can be admininstered from 2 months of age. If there is any concern regarding a possible diagnoses of an IFNAR1 deficiency it is safe to proceed with JEspect in preference to the live-attenuated IMOJEV (licensed from 9 months of age).
 

Injection site reactions and minor systemic reactions are common after Japanese encephalitis vaccination.

Adverse events following Imojev

In adults

In studies of adults, adverse events after receiving Imojev were similar to those in placebo recipients,^6,16 but occurred less often than in recipients of JE-Vax.¹⁶ The most common adverse events in 2 key studies were:^6,16

• injection site pain
• headache
• fatigue
• malaise

Most symptoms resolved within 3 days.¹⁶

In children

In studies of children aged 12 to 24 months, the frequency of adverse events after receiving Imojev was similar to that after the hepatitis A vaccine. About 40% of children reported injection site reactions, including:¹⁷

• pain (32%)
• redness (23%)
• swelling (9%)

About 50% reported at least 1 systemic reaction, including:¹⁷

• fever (21%)
• appetite loss (26%)
• irritability (28%)
• abnormal crying (23%)

In children aged 2–5 years who received Imojev after JE-Vax, the frequency of adverse events was similar to, or lower than, that in children aged 12–24 months who had not previously been vaccinated. All reactions were transient, and almost all were mild. Most systemic reactions were mild or moderate, appeared within 7 days of vaccination, and lasted for up to 3 days.¹⁷

Adverse events following JEspect

In a pooled analysis of more than 4000 healthy adults who received at least 1 dose of JEspect, 54% reported injection site reactions, most commonly: ¹⁸

• pain (33%)
• tenderness (33%)
• redness (9%)

Headache and myalgia were the most commonly reported systemic adverse events.^18-22

Another analysis compared adverse events after receiving JEspect vaccine and aluminium-containing placebo. The rate of adverse events was similar.²¹

Post-marketing surveillance reported adverse events after receiving JEspect at a rate of about 10 per 100,000 doses distributed. No serious allergic reactions were observed during the first 12 months after marketing approval.¹⁸

Frequencies of adverse events reported following a booster dose of JEspect were similar to those reported after a primary course.^19,23

Japanese encephalitis (JE) is caused by a mosquito-borne RNA flavivirus called Japanese encephalitis virus.

Pathogenesis

After an infectious mosquito bite, the virus multiplies locally and in regional lymph nodes. The virus then spreads to secondary sites, particularly the central nervous system.²⁴

Transmission

JE is a zoonosis of pigs and wading birds. Culicine mosquitoes transmit the virus between these animals.¹

Pigs and wading birds are ‘amplifying hosts’ for JE virus. This means that the virus replicates in the animal and causes transient high-level viraemia. Other large vertebrates, such as horses and humans, are not amplifying hosts. Humans are an incidental host.

Asymptomatic infection

Most infections are asymptomatic. Estimates of the symptomatic to asymptomatic infection ratio vary in different populations from 1:25 to 1:1000.¹

Acute neurological illness

Japanese encephalitis (JE) is typically an acute neurological illness. It is characterised by: ²⁵

• headache
• fever
• convulsions
• focal neurological signs
• depressed level of consciousness

JE has a high case-fatality rate of around 30%. There is no specific treatment. Around 50% of people who survive the acute illness will have neurological sequelae.¹

JE sometimes presents as acute flaccid paralysis.¹ Milder forms of JE include febrile illness with headache, and aseptic meningitis.²⁴

JE in Torres Strait and mainland Australia

Occasional outbreaks of Japanese encephalitis (JE) have occurred in Torres Strait.^26,27 The first outbreak of JE in Australia occurred in the remote outer islands of Torres Strait in 1995. There were 3 cases, including 2 deaths.²⁸

A second outbreak of JE occurred in 1998.²⁹

More recently, a JE outbreak was reported on mainland Australia, with 46 human cases and seven deaths reported between March 2021 and February 2023. Japanese encephalitis virus was also detected in piggeries, mosquitoes and sentinel chickens during this period, and is believed to indicate ongoing JEV transmission in south-eastern Australia.³⁰

Torres Strait has not reported a case of JE since 1998. The risk of JE is much lower now than it was in the mid-1990s. Most communities have moved pigs well away from homes. Major drainage works on most islands have reduced breeding sites for mosquitoes.

Between 2001 and 2014, 9 cases of JE were notified in Australia. All were acquired overseas.^31,32

JE in Asia

Japanese encephalitis (JE) is a significant public health problem in many parts of Asia, including:^1,24

• China
• the Indian subcontinent
• Southeast Asia

The disease is also present in eastern Indonesia (including Bali) and Papua New Guinea.

JE virus is maintained in an enzootic cycle. Areas that have established JE immunisation programs may not have many cases in humans, but susceptible people who visit these areas may still be at risk.

People are most commonly infected if they live close to amplifying hosts and mosquitoes. This usually occurs in rural areas where the mosquito vectors can breed in flooded rice fields.¹ Incidence of JE has declined considerably in Japan, Taiwan, South Korea and some provinces of China. Singapore has eradicated the disease. These changes can be attributed to:¹

• immunisation
• changes in pig husbandry
• less land used for rice farming
• improved socioeconomic circumstances

Up-to-date information about JE virus activity in specific locations is available from sources such as the Centers for Disease Control and Prevention (CDC) Yellow Book.⁴

Seasonal patterns of JE

In temperate or subtropical regions of Asia, JE mainly occurs in epidemics during the summer or wet season. This is usually April–May to September–October.

In tropical regions, the disease occurs throughout the year, but is more prominent during the wet season.¹

Two Japanese encephalitis (JE) vaccines are available for use in Australia: Imojev and JEspect. They have different:

• registered age groups
• vaccination schedules
• booster dose requirements
• contraindications for use

Consider these factors when deciding the most appropriate vaccine.

See Contraindications and precautions for more details.

Imojev is produced using recombinant technology using genes from the SA 14-14-2 JE virus strain. It is:

• Vero cell derived
• live attenuated

JEspect is based on the attenuated SA 14-14-2 JE virus strain. It is:

• Vero cell derived
• inactivated
• aluminium adjuvanted

The World Health Organization considers that a neutralising antibody titre of ≥1:10 correlates to immunity. Imojev and JEspect were registered based on this immunologic correlate rather than efficacy trials.

Imojev

Immunogenicity in adults

1 dose of Imojev:

• causes seroconversion to a homologous JE virus strain in about 94% of healthy adults aged 18–84 years within 14 days¹⁶
• causes seroconversion to various wild-type, non-homologous JE virus strains in 70–100% of adults^17,33
• generates protective levels of neutralising antibodies against the homologous vaccine virus strain in 99% of adults after 28 days^6,16,17,34
• generates protective levels of neutralising antibodies against all 4 wild-type strains used for testing in 85% of adults⁶

Duration of immunity in adults

Protective antibodies against the vaccine strain were maintained in:⁶

• 98% of adults at 1 year after vaccination
• 92% at 3 years after vaccination
• 87% at 5 years after vaccination

About 65% of adults maintain protective antibody levels to at least 3 wild-type virus strains 5 years after a single vaccine dose.⁶ Studies also show immunological memory in vaccinated adults.³³

Immunogenicity in children

In vaccine-naive children aged 12–24 months, 1 dose of Imojev induces protective levels of neutralising antibodies against the homologous vaccine virus strain in:

• 96% at 28 days¹⁷
• 87% at 7 months¹⁷
• 75% at 3 years³⁵

70–97% of children aged 12–24 months seroconverted to various wild-type, non-homologous, JE virus strains.^17,33

In children aged 2–5 years who previously received JE-Vax, 86% were seropositive against the vaccine strain at baseline, and 72–81% were seropositive against 4 wild-type strains at baseline. A dose of Imojev produced seroprotective antibody levels against the vaccine strain in 100% of the children, and against all wild-type strains in 99%. Of these children:

• 93% seroconverted to the vaccine strain
• 90% seroconverted to all wild-type strains¹⁷

No immunogenicity data are available for Imojev in children aged 6–17 years. Immunogenicity in this age group is assumed based on studies in younger children and adults.

Duration of immunity in children

In children aged 9–18 months, protective levels of neutralising antibodies were present 12 months after a single dose of Imojev.³⁶ After a booster dose in these children, 100% demonstrated seroprotection after 28 days and 98% remained seroprotected after 5 years.³⁷

In children aged 2–5 years who previously received JE-Vax, 98% maintained protective antibody levels for 3 years after a dose of Imojev.³⁵

JEspect

Immunogenicity in adults

A randomised controlled trial conducted among more than 860 adults using neutralising antibody responses 1 month after the last vaccine dose showed that 98% of people had seroconverted.³⁸

A small study looked at travellers who had received at least 2 doses of JE-Vax 1–21 years before a dose of JEspect. 4–8 weeks after 1 dose of JEspect, 98% of people had protective antibody levels against homologous strains and 95% against heterologous strains.¹²

Accelerated JEspect schedule in adults

A phase III study in healthy adults compared an accelerated (days 0 and 7) and conventional (days 0 and 28) JEspect schedule. The results suggested a non-inferior immune response for the accelerated schedule of JEspect compared with the conventional schedule. This was at 28 days and 1 year after the 2nd dose. ^39,40

Duration of immunity in adults

Table. Protective neutralising antibody levels for JEspect in vaccinated people in Europe shows the protective levels of neutralising antibodies after 2 doses of JEspect. The 2 studies show a discrepancy, especially in the first 12 months. This may be because of previous vaccination with the tick-borne encephalitis (TBE) vaccine in a large proportion of people in the central European study.^19,41

Another study looked at JEspect-vaccinated people after 5 years.⁴³ The seroprotection rates were:

• 64% for TBE-naive people
• 92% for those who received TBE vaccine before or during the study

The western and northern European study also reviewed JE booster doses. People who did not have a seroprotective antibody level at the 6- or 12-month follow-up received a booster dose at 11 and/or 23 months after 1st vaccination. 100% of these people became seropositive.¹⁹

Immunogenicity in children

The key paediatric clinical trial of JEspect was a phase III study in children aged 2 months to 17 years in the Philippines.^44,45

Children received age-appropriate doses of JEspect, and the proportions with protective antibody titres were:⁴⁵

• 99–100% after 56 days
• 85–100% after 7 months

There was no obvious pattern by age.

A phase III immunogenicity study involved 64 children from non-JE-endemic countries aged 2 months to <18 years (mean 11.6 years; range 11 months to 17.9 years). All children seroconverted to protective levels. The proportion seroprotected at 6 months after the 2nd dose was:⁴⁶

• 100% in the <3 years age group (n = 2)
• 90.6% in the ≥3 years age group (n = 32)

Transport according to National vaccine storage guidelines: Strive for 5.⁴⁷ Store at +2°C to +8°C. Do not freeze. Protect from light.

Imojev vaccine must be reconstituted. Add the entire contents of the diluent container to the vial and shake until the powder completely dissolves. Use the reconstituted vaccine within 1 hour.

Japanese encephalitis virus infection is a notifiable disease in all states and territories in Australia.

State and territory public health authorities can provide advice about the public health management of Japanese encephalitis, including management of cases.

The product information for JEspect states that this vaccine is for use in people aged ≥18 years.

The Australian Technical Advisory Group on Immunisation (ATAGI) recommends that children and adolescents aged ≥2 months to <18 years can receive JEspect if an alternative is not available or is contraindicated. ATAGI also recommends that children aged ≥2 months to <3 years receive 0.25 mL doses of JEspect.

The product information for JEspect states that it can be given with inactivated hepatitis A vaccine.

ATAGI recommends that JEspect can also be given with quadrivalent (ACWY) meningococcal conjugate vaccine and rabies vaccine (see Vaccines, dosage and administration).

The product information for Imojev states that this vaccine is to be adminstered via the subcutaneous route. 

ATAGI recommends that Imojev can be given intradermally in special circumstances where dose sparing may be required, such as in the context of outbreaks, expanded eligibility in risk areas, and ongoing supply considerations.⁵

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