What is it?

Japanese encephalitis virus (JEV) is a rare but potentially serious infection caused by the flavivirus and is spread to humans through mosquito bites. It is a leading cause of vaccine-preventable encephalitis (brain infection) in Asia and the Western Pacific. It has recently been detected in piggeries in New South Wales, Queensland and Victoria and human cases have been reported.

What to look for?

Most people infected with JEV do not develop symptoms. Symptoms occur 5-15 days after exposure and can be non-specific or mild and can include fever, diarrhoea and rigors. Less than 1% of infected people develop encephalitis. Symptoms of encephalitis may include headache, confusion, vomiting, weakness, seizures or neck stiffness. In severe cases resulting in encephalitis there is a mortality rate of 20-30%, with permanent neurological disability occurring in 30-50% of survivors.

How is it transmitted?

JEV is spread by mosquito bites. It mostly affects pigs, horses and water birds, but can also cause disease in humans. It cannot be transmitted from person to person, or through eating pork or pig products. When JEV occurs during pregnancy, infection can also be transmitted to the unborn baby.

Epidemiology

People with increased exposure to mosquitos may be at higher risk of infection, particularly those spending time outdoors or working with pigs. Children under 5 years of age and older people who are infected with JEV are at higher risk of more severe illness, including encephalitis.

JEV is very common across Asia and the Western Pacific. As of March 4th, 2022 an outbreak in the eastern states of Australia was identified with a likely increased risk of transmission near the border of New South Wales and Victoria, particularly in the Murray River region.

Prevention

Preventing mosquito bites with physical barriers is the most effective way to avoid infection with JEV. Vaccines are also available to prevent the development of disease.

Physical barriers

Exposure to mosquito bites can be prevented in a number of ways including:

  • wearing long, loose fitting clothes when outdoors
  • using mosquito repellents containing picaridin or DEET
  • limiting outdoor activity if there are lots of mosquitoes
  • using fly sprays, mosquito coils and plug-in repellent devices
  • sleeping under mosquito nets treated with insecticides if you don’t have flywire screens on your windows or are sleeping in an untreated tent/in the open outdoors.

Vaccines

There are currently two JEV vaccines available in Australia, Imojev and JEspect.

  • Imojev

    Imojev
    Age Route Dose Primary course Booster dosesΩ
    ≥ 9 months – < 18 years SC 0.5ml 1 doseß Single age- appropriate booster dose of either JEspect§/Imojevß is recommended if ≥ 12 months has passed since the completion of the primary course and the individual has ongoing risk of JEV exposure.
    ≥ 18 years SC 0.5ml 1 doseß Individuals who received a primary course of Imojevß at ≥ 18 years do not require a booster dose, regardless of ongoing risk of exposure.

    Ω For further information on booster doses refer to ATAGI clinical guidance on Japanese encephalitis virus vaccines.
    ß Imojev is a live-attenuated vaccine and therefore is contraindicated for pregnant women or people who are immunocompromised.
    § JEspect is an inactivated vaccine and is therefore suitable for administration in pregnant women or immunocompromised individuals.

  • JEspect

    JEspect
    Age Route Dose Primary course Booster dosesΩ
    ≥ 2 months  – < 3 years IM 0.25ml 2 doses (28 days apart) Single age- appropriate booster dose of either JEspect§/Imojevß is recommended if ≥ 12 months has passed since the completion of the primary course and the individual has ongoing risk of JEV exposure.
    ≥ 3 years – < 18 years IM 0.5ml 2 doses (28 days apart) Single age- appropriate booster dose of either JEspect§/Imojevß is recommended if ≥ 12 months has passed since the completion of the primary course and the individual has ongoing risk of JEV exposure.
    ≥ 18 years IM 0.5ml 2 doses (28 days apart)δ Single age- appropriate booster dose of either JEspect§/Imojevß is recommended if ≥ 12 months has passed since the completion of the primary course and the individual has ongoing risk of JEV exposure^.

    Ω For further information on booster doses refer to ATAGI clinical guidance on Japanese encephalitis virus vaccines.
    ß Imojev is a live-attenuated vaccine and therefore is contraindicated for pregnant women or people who are immunocompromised.
    § JEspect is an inactivated vaccine and is therefore suitable for administration in pregnant women or immunocompromised individuals.
    δ The interval can be shortened to 7 days for those aged ≥ 18 years if the individuals is at imminent risk of JEV exposure.
    ^ booster doses are not required for individuals who received a primary course of Imojev at ≥18 years of age, regardless of ongoing risk of exposure.

Side effects following either vaccine are usually mild and short-lived. They can include injection site pain, redness and swelling, as well fever, headaches and muscle aches.

Australian outbreak advice- March 2022

Following reported cases of JEV in the eastern states of Australia, ATAGI and CDNA have provided advice on JEV vaccine distribution.

The following groups on mainland Australia, Torres Strait Islands and Tiwi Islands have been identified for priority JEV vaccination with the current supply of vaccines:

  • people who work at, reside at or are visiting a piggery, pork abattoir or pork rendering plant (including children ≥ 2 months of age)
  • personnel who work directly with mosquitoes through surveillance, control and management, and indirectly through management of vertebrate mosquito-borne disease surveillance systems.
  • all diagnostic and research laboratory workers who may be exposed to JEV.

Further information can be found via the common queries below:

  • Should individuals travelling to the Murray River region receive the JEV vaccine?

    The current supply of JEV vaccine is currently being prioritised in Australia for high-risk groups, especially those who work or live at piggeries, or who work with mosquitoes and laboratory staff working with JEV.

    Individuals travelling to regions associated with JEV cases for holiday purposes are recommended to take additional precautions to avoid mosquito contact.

  • What is the difference between the two JEV vaccines and can individuals choose which JEV vaccine they receive?

    There are two JEV vaccines available in Australia:

    • Imojev is a live-attenuated vaccine which can be given as a single dose from ≥ 9 months of age. It cannot be administered in pregnant women or those with immunocompromise.
    • JEspect is an inactivated vaccine given in a two-dose schedule and is registered for use in those ≥ 2 months of age. It is suitable for administration in immunocompromised individuals and pregnant women.

    Vaccine choice will be determined by age, immune-status, pregnancy and availability.

  • How effective are JEV vaccines?

    Around 95% of people develop a protective immune response to the virus around 28 days after receiving their vaccine.

    Other measures such as physical barriers and effective mosquito repellent are also recommended to avoid exposure.

  • Can pregnant women receive JEV vaccines?

    Women who are pregnant are recommended to receive JEspect as it is an inactivated JEV vaccine.

    Imojev is a live-attenuated vaccine and is therefore contraindicated in pregnancy. Women should be advised to avoid falling pregnant for 28 days following vaccination with Imojev. Pre-immunisation screening is essential to identify pregnant women.

    NB: JEV infection occurring in the first and second trimesters has been associated with miscarriage.

  • Can women who are breastfeeding receive JEV vaccines?

    JEspect is the preferred JEV vaccine for breastfeeding women.

    There is minimal data on the safety of Imojev when administered to breastfeeding women and therefore it is not routinely recommended. In women with a high risk of mosquito exposure, Imojev may be considered in circumstances where JEspect is unavailable and following consultation with a specialist immunisation service.

  • Can immunocompromised individuals receive JEV vaccines?

    People who are immunocompromised can receive JEspect.

    Imojev is a live-attenuated vaccine and is therefore contraindicated in this patient group due to the risk of vaccine-associated disease.

  • Are booster doses of JEV vaccines recommended?

    Yes. A single age-appropriate booster dose of either JEspect or Imojev is recommended for individuals if ≥ 12 months has passed since the completion of the primary course and the individual has an ongoing risk of JEV exposure. This recommendation also applies to individuals who completed their primary course overseas with a brand not available in Australia (eg. JEvax).

    The only exception to this is individuals received a primary course of Imojev when the individual was ≥ 18 years of age. In these circumstances, further booster doses are not required.

Resources

Authors: Rachael Purcell (Immunisation Fellow, Royal Children’s Hospital), Nigel Crawford (Director, SAEFVIC, Murdoch Children’s Research Institute), Rachael McGuire (MVEC Education Nurse Coordinator) and Raffaela Armiento (Consultant Paediatrician, Victorian Immunisation Specialist Services)

Reviewed by: Rachael Purcell (Immunisation Fellow, Royal Children’s Hospital) and Rachael McGuire (MVEC Education Nurse Coordinator)

Date: March 30, 2022

Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family’s personal health. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult a healthcare professional.