Traveller vaccination japanese encephalitis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1];Associate Editor(s)-in-Chief: Seyedmahdi Pahlavani, M.D. [2]

Disease cause

Japanese encephalitis virus

Transmission

Pigs and various wild birds represent the natural reservoir of this virus, which is transmitted to new animal hosts and occasionally human beings by mosquitoes of the genus Culex.
Culex mosquitoes are primarily day-biting.

Nature of the disease

Mostly asymptomatic. Symptoms could be vary from mild infections that are characterized by febrile headache or aseptic meningitis followed by an uneventful recovery; severe cases have a rapid onset and progression with headache, high fever and meningeal signs.

Geographical distribution

Japanese encephalitis virus is the leading cause of viral encephalitis in Asia and occurs in almost all Asian countries. Transmission occurs mainly during the rainy season in south-east Asia but may take place all year round, particularly in tropical climate zones.

Risk for travellers

The risk of japanese encephalitis is very low for most travellers to Asia, particularly for short-term visitors to urban areas. However, the risk varies according to season, destination, duration of travel and activities.

Vaccination is recommended for travellers with extensive outdoor exposure (such as camping, hiking and working) during the transmission season, particularly in endemic countries or areas where farming involves flooding irrigation.
Prevention is by avoidance of mosquito bites and by vaccination.

Vaccine

Inactivated Vero cell-derived, live attenuated and live recombinant vaccines are available. Vaccination against japanese encephalitis is recommended for travellers to endemic areas who will have extensive outdoor exposure during the transmission season.

Summary of vaccine data

Considerations
Type of vaccine and schedules	Japanese encephalitis vaccines fall into 3 classes:
	Inactivated Vero cell-derived vaccines: It requires 2 intramuscular doses administered 4 weeks apart. The recommended age for the first dose varies. The dose for children aged <3 years is 0.25 ml, and for those aged ≥3 years 0.5 ml. Travellers aged ≥17 years who have received primary immunization more than one year previously may be given a booster dose if continued or repeated exposure to Japanese encephalitis virus is expected.
	Live attenuated vaccines: In China, the first dose is given subcutaneously at age 8 months, followed by a booster dose at 2 years of age. In Australia, the first dose is administered to persons aged ≥ 9 months, and no booster is required for persons aged ≥18 years when receiving the primary dose. In some areas and countries, an additional booster is offered at 6–7 years of age.
	Live recombinant (chimeric) vaccines: Primary immunization is with 1 dose given subcutaneously at 9 months of age or older. A booster dose is recommended 12–24 months later for those <18 years of age. Currently, there is no booster recommendation for adults.
Adverse reactions	Occasional mild local or systemic reactions.
Contraindications and precautions	A hypersensitivity reaction to a previous dose is a contraindication. In principle, the live attenuated vaccine should not be given to pregnant women unless there is a high risk of exposure to the infection.