Japanese Encephalitis (JE) is an acute viral infection caused by the Japanese encephalitis virus, transmitted principally via the bite of infected mosquitoes (primarily Culex tritaeniorhynchus). The virus primarily amplifies in vertebrate hosts such as pigs before spreading through mosquito vectors; it is not transmitted directly from person to person.

Whilst the majority of infections are asymptomatic or present with mild symptoms such as fever and headache, viral invasion of the central nervous system can cause severe encephalitis and permanent neurological impairment. Severe cases carry a case fatality rate of up to 30%, and 30% to 50% of survivors experience permanent neurological, cognitive, or behavioural sequelae (such as seizures, hearing or visual impairment, speech difficulties, and motor weakness). As there is no specific antiviral treatment and the vaccine is not part of the routine public immunisation schedule, individuals requiring protection must arrange it proactively through private healthcare services.

Protective Efficacy of the Japanese Encephalitis Vaccine (Imojev)

The Japanese encephalitis vaccine (Imojev) can effectively stimulating the immune system to produce virus-neutralising antibodies for long-lasting protection.

• Vaccine Manufacturer: Substipharm Biologics (France)
• Clinical Significance: Vaccination significantly lowers the risk of contracting Japanese encephalitis and developing severe, debilitating neurological disease.
• Recommended Immunisation Schedule:
  • Individuals Aged 9 Months to 17 Years: A 2-dose primary series is required (with the second dose administered 12 to 24 months after the first for complete protection).
  • Individuals Aged 18 Years and Older: A single dose is required.
• Target Demographic: Suitable for individuals aged 9 months and older who plan to travel, work, or engage in outdoor activities in Japanese encephalitis-endemic regions (such as rural, agricultural, and paddy field areas across Asia and the Western Pacific).

Understanding Japanese Encephalitis — Risks and Clinical Features

Japanese encephalitis is endemic in many parts of Asia and the Western Pacific (including Mainland China, Japan, South Korea, Southeast Asia, and India). Understanding its transmission helps in effective prevention:

• Transmission Routes: Spread exclusively through the bite of infected Culex tritaeniorhynchus mosquitoes. Pigs serve as primary amplifying hosts; mosquitoes acquire the virus from infected animals and transmit it to humans.
• High-Risk Groups:
  • The majority of natural clinical cases occur in children under 15 years of age.
  • Travellers, expatriates, or students planning visits to endemic regions (such as areas near paddy fields, pig farms, or rural districts).
  • Individuals working outdoors or participating frequently in camping, hiking, or other outdoor recreational activities.
• Key Clinical Manifestations & Incubation Period:
  • The incubation period typically ranges from 4 to 14 days.
  • Early Symptoms: Fever, chills, headache, nausea, vomiting, and abdominal discomfort, closely resembling common viral illnesses.
  • Severe Encephalitis Signs: Once the virus invades the brain, it triggers persistent high fever, neck stiffness, altered mental status, disorientation, coma, convulsions, and spastic paralysis.

Safety, Adverse Reactions, and Contraindications

The Japanese encephalitis vaccine is generally safe and effective, though mild transient reactions may occur:

• Common Mild Reactions: Following subcutaneous injection, local pain, erythema, swelling, or induration may occur at the injection site. Some recipients may experience transient low-grade pyrexia, lethargy, myalgia, or headache, which typically resolve spontaneously within a few days.
• Contraindications and Precautions:
  • Individuals currently suffering from moderate or severe acute febrile illnesses (postponed until recovery).
  • Individuals with a history of severe hypersensitivity or anaphylaxis to any component of the vaccine.
  • Individuals with immunosuppressive conditions or undergoing immunosuppressive therapy.
  • Pregnant or breastfeeding women.
(Please consult a registered medical practitioner for tailored advice based on your health status.)

Frequently Asked Questions