Japanese encephalitis

What

Japanese encephalitis (JE) is a rare but serious illness 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 anyone in a JE risk region of Australia who may be bitten by mosquitoes (see separate recommendation for advice on Torres Strait)
• routine vaccination of laboratory workers who may be exposed to JE virus
• routine vaccination of travellers to endemic areas outside Australia during the JE virus transmission season
• routine vaccination of people who live or work on the outer islands of Torres Strait

How

Two vaccines are available in Australia to protect people who are at risk of exposure to JE virus: Imojev live JE vaccine and JEspect inactivated JE vaccine.

Both vaccines are safe and effective. Imojev is contraindicated in pregnant women and immunocompromised people and is not registered for use in children <9 months of age.

People who are at ongoing risk of being infected by JE virus may need booster doses.

Why

A number of JE cases have been reported in mainland Australia since 2022, and ongoing JE virus transmission is occurring in some states and territories. JE is a significant public health concern in many parts of Asia, and occasional outbreaks have occurred in Torres Strait. JE has a high case fatality rate of around 30% in symptomatic cases. 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. Subcutaneous administration is optimal and is the preferred route of administration.

Imojev may be given intradermally for dose-sparing purposes in settings where public health benefit outweighs potential risk, for example, in outbreak risk populations where vaccine supply would otherwise be insufficient. See the ATAGI statement on the intradermal use of Imojev Japanese encephalitis vaccine.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 0.5 mL. Immunisers should draw up 0.25 mL and discard the remaining half dose.

Children aged ≥3 years and adults should receive 2 doses, each of 0.5 mL.

The interval between the 2 doses of JEspect for the primary course should be 7 days for adults who are at a high risk of immediate exposure (e.g. residents of, or farm workers on, affected piggeries).

The interval between the 2 doses of JEspect for the primary course should be 4 weeks for adults at lower risk of exposure and for all children (aged ≥2 months to <18 years).

Co-administration with other vaccines

Imojev

People can receive Imojev at the same time as:

• yellow fever vaccine⁶
• MMR (measles-mumps-rubella) vaccine⁷

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 any other vaccines are available, but Imojev can be co-administered with other vaccines, including seasonal influenza vaccine, if required.

If co-administering a JE vaccine with another vaccine, it is recommended to administer the vaccines in different limbs.

JEspect

People can receive JEspect at the same time as:

• hepatitis A vaccine⁸
• quadrivalent (ACWY) meningococcal conjugate vaccine⁹
• rabies vaccine⁹

Co-administration with other vaccines (including seasonal influenza vaccine or yellow fever vaccine) has not been assessed, but JEspect can be co-administered with other vaccines, if required.

If co-administering a JE vaccine with another vaccine, it is recommended to administer the vaccines in different limbs.

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 and require vaccination. ^10-12

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 (see Vaccination for 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.¹³

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 as a result of a primary/secondary medical condition(s), or receiving certain immunosuppressive therapies (e.g. B-cell depleting biological therapy). However, there are few data on the safety and efficacy of JEspect in those who are immunocompromised. These individuals may not mount an adequate immune response to the vaccine.

See Table. Types of medical conditions and immunosuppressive therapy and associated levels of immunocompromise in Vaccination for people who are immunocompromised.

Women who are pregnant or breastfeeding

Pregnant women requiring JE vaccination 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 requiring JE vaccination are recommended to receive JEspect. However, no specific data are available about using JEspect in breastfeeding women. Although JEspect is preferred, Imojev is not considered contraindicated in breastfeeding by the Australian Technical Advisory Group on Immunisation (ATAGI).

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.

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

Adverse events following Imojev

In adults

In studies of adults, adverse events after receiving Imojev were similar to those in placebo recipients,^14,15 but occurred less often than in recipients of JE-Vax.¹⁴ The most common adverse events in 2 key studies were:^14,15

• injection site pain
• headache
• fatigue
• malaise

Most symptoms resolved within 3 days.¹⁴

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:¹⁶

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

About 50% reported at least 1 systemic reaction, including:¹⁶

• 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.¹⁶

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: ¹⁷

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

Headache and myalgia were the most commonly reported systemic adverse events.^17-21

Another analysis compared adverse events after receiving JEspect vaccine and aluminium-containing placebo. The rate of adverse events was similar.²⁰

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.¹⁷

Frequencies of adverse events reported following a booster dose of JEspect were similar to those reported after a primary course.^18,22

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.²³

Transmission

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

Water birds are considered maintenance hosts and pigs are amplifying hosts for JE virus. Following infection, the virus replicates in these vertebrate hosts and causes transient high-level viraemia capable of infecting biting mosquitoes. Other large vertebrates, such as horses and humans, are not amplifying hosts. Humans are an incidental host.

JE virus is maintained in an enzootic cycle between birds, pigs and mosquitoes. Areas with established JE immunisation programs may not report many cases in humans, but susceptible people who live in or visit these areas may still be at risk.

People are most commonly infected if they live close to amplifying hosts (pigs and wading birds) and mosquitoes. This usually occurs in rural areas where the mosquito vectors use fresh water in flooded ground pools as larval habitats.¹ The incidence of JE has declined considerably in Japan, Taiwan, South Korea and some provinces of China. Singapore has eliminated the disease. These changes can be attributed to:¹

• immunisation
• changes in pig husbandry
• changes in rice farming practices
• 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

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: ²⁴

• 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.²³

JE in Torres Strait and mainland Australia

Occasional outbreaks of Japanese encephalitis (JE) have occurred in Torres Strait.^25,26

The first outbreak of JE in Australia occurred in the remote outer islands of Torres Strait in 1995, with three cases, including two deaths.²⁷ A second outbreak occurred in 1998.²⁸ Torres Strait has not reported a case of JE since 1998. Risk has been mitigated through vaccination programs, changing pig farming practices and major drainage works, reducing mosquito breeding sites.

In 2022, a JE outbreak was identified in the south-eastern states of mainland Australia. Fifty-one human cases and eight deaths were reported between 2021 and early 2025. Japanese encephalitis virus has also been detected in piggeries, mosquitoes and sentinel chickens during this period, indicating ongoing JE virus transmission in mainland Australia.²⁹ As of early 2025, cases have been reported in New South Wales, Victoria, Queensland, the Northern Territory and South Australia. Risk regions are likely to change depending on weather events such as flooding, and movement of animal vectors. Older males have been the group with the highest number of cases, but cases have also occurred in women, children and younger people.³⁰

Since 2022, the JE cases in Australian temperate regions have mostly been reported during the warmer months, typically from December to March.

Seventeen overseas-acquired cases of JE were notified in Australia between 2000 and 2020.³⁰

JE in Asia

JE is a significant public health problem in many parts of Asia, including:^1,23

• China
• the Indian subcontinent
• Southeast Asia

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

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 Japanese Encephalitis vaccines, Contraindications and precautions for more details.

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 days¹⁴
• causes seroconversion to various wild-type, non-homologous JE virus strains in 70–100% of adults^16,31
• generates protective levels of neutralising antibodies against the homologous vaccine virus strain in 99% of adults after 28 days^14-16,32
• generates protective levels of neutralising antibodies against all 4 wild-type strains used for testing in 85% of adults.¹⁵

Duration of immunity in adults

Protective antibodies against the vaccine strain were maintained in:¹⁵

• 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.¹⁵ Studies also show immunological memory in vaccinated adults.³¹

Immunogenicity in children

In phase III studies of vaccine-naive children, 1 dose of Imojev induced protective levels of neutralising antibodies against the homologous vaccine virus strain, achieving seroprotection in:

• 95%-100% of children aged 9-24 months at 28 days^16,33-35
• 95% of children aged 9-18 months at 6 months³³
• 87% of children aged 12-24 months at 7 months¹⁶

70–97% of children aged 12–24 months seroconverted to various wild-type, non-homologous, JE virus strains.^16,31

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

Duration of immunity in children

In a randomised trial studying primary immunisation with Imojev in children aged 9–18 months, 88% of children had protective levels of neutralising antibodies 12 months after a single dose of Imojev.³³  

In a study of Thai children aged 12-24 months, 84% had protective levels of vaccine-induced antibodies 12 months after a single dose of Imojev.¹⁶

In another study of Thai children who were given a primary dose at 12-18 months of age, seroprotection waned to 82.9% at 2 years and 68.6% at 5 years.¹³ No long-term immunogenicity data are available beyond 5 years after the administration of a single dose of Imojev.¹³

Booster doses

In a study conducted in the Philippines, 100% of children aged 36-42 months demonstrated seroprotection immediately after a booster dose given 2 years after a primary dose, and 98% remained seroprotected after 5 years.³⁶

Immunogenicity after intradermal vaccination

An Australian randomised controlled trial of a single 0.1 mL intradermal dose of Imojev was conducted among 236 study participants aged ≥5 years. Seroconversion after 1 month measured by plaque reduction neutralisation serum antibody titres showed a non-inferior response when comparing intradermal (96.5% [95% CI: 91.2%, 99.0%]) to subcutaneous route (99.2% [95.4%, 100.0%]). Overall seropositivity after 6 months was 95.6%, with no significant difference between the intradermal (97.3% [92.0%, 97.9%]) and subcutaneous (94% [88.0%, 97.5%]) routes.³⁷

JEspect

Immunogenicity in adults

Data from randomised controlled trials conducted in adults in Europe showed that 97-99% of people had seroconverted 1 month after primary vaccination.^22,38-41 A study in older adults aged 64-83 years old showed a lower seroprotection rate of 65% 2 months after vaccination.⁴²

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 measured at 28 days and 1 year after the second dose.^43,44

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 in seroprotection rate after 12 months. This may be because of previous vaccination with the tick-borne encephalitis (TBE) vaccine in a significant proportion of people in the central European study.⁴⁵

*68% in subjects not previously vaccinated for TBE compared to 90% in subjects with previous TBE vaccination

A 5-year follow-up from one central European study also stratified participants based on their TBE vaccination status. It showed seroprotection rates of 64% in TBE-naive people and 86% in those who had previously received TBE vaccine.³⁹

Booster doses

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 either at 11 or 23 months after primary vaccination, both resulting in 100% seroconversion.¹⁸ Another small study in central Europe showed persistent seroprotection of 96% at 6 years after a booster dose given 15 months after primary vaccination.⁴⁶

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.⁴⁷ Children received age-appropriate doses of JEspect, and the proportions with protective antibody titres were:

• 99–100% after 56 days
• 86–100% 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:⁴⁸

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

Duration of immunity in children

Long-term immunogenicity data for a primary course of JEspect in children are limited. The phase III study in the Philippines found an 89.9%⁴⁹ seroprotection rate 12 months after the primary course, which remained at 89% and 90% at 2 and 3 years, respectively (unpublished data from European Medicines Agency JEspect Product Information).⁵⁰

Booster doses

The same Philippine study assessed the effects of a booster dose given 12 months after primary vaccination in children (median age of 3 years, ranging from 1.6 to 17 years). The booster led to a pronounced increase in geometric mean titer and 100% seroprotection rate in all age groups 2 years after the booster.⁴⁹

Transport according to National vaccine storage guidelines: Strive for 5.⁵¹ 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 (JE) 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 JE, 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 Imojev recommends that from 12 months of age, Imojev may be administered at the same time as vaccines against measles, mumps, or rubella, either standalone or combined, and that for children living in or travelling to areas where risk for measles is high, Imojev may be administered at the same time as measles vaccine, either stand alone or combined with mumps and/or rubella vaccines, from as early as 9 months of age. The product information recommends Imojev may be administered to adults at the same time as the yellow fever vaccine.

ATAGI recommends that Imojev can be co-administered with any other vaccine, including seasonal influenza vaccine, if required.

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

ATAGI recommends that Imojev can be given intradermally as a dose-sparing strategy in settings where public health benefit outweighs potential risks, for example in outbreak risk populations where vaccine supply would otherwise be insufficient.⁵

The product information states that Imojev is contraindicated in breastfeeding.

ATAGI recommends that although JEspect is the preferred vaccine where JE vaccination is indicated in breastfeeding women, Imojev is not contraindicated in breastfeeding.

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