Japanese B Encephalitis Vaccination

Japanese Encephalitis-VAX has been produced since 1992 by BIKEN (Osaka, Japan). It is a formalin-inactivated vaccine derived from mouse brain.

It is not available on the NHS.
It is an unlicensed vaccine but can be obtained on a named patient basis from Sanofi Pasteur and MASTA (Medical Advisory Services for Travellers Abroad).

The other flava virus vaccine is yellow fever vaccine. There does not appear to be cross protection with other flava virus disease, although the risk of Dengue may be reduced.

After 3 doses of the vaccine the seroconversion rate appears to be 100%.¹ Immunisation regimens use 3 doses for those coming from outside the area but just 2 doses are given to those normally resident in endemic areas.
Efficacy is good but not 100%. In an early trial involving 65,00 children in Thailand who received either 2 doses of JE vaccine or tetanus toxoid as control, the attack rate was 51 per 100,000 in the control group compared with 5 per 100,000 in the active group.²

• The current schedule for those aged 3 years or older is 1.0 ml subcutaneously on days 0, 7, and 30.
• For those aged 1 and 2 the dose is 0.5ml.
• A schedule of 0, 7, and 14 days may be used if rapid immunization is required. The shorter schedule tends to produce lower titres at 2 and 6 months but the seroconversion rate remains very high.¹ Whether or not this represents a lower level of protection is unclear.
• The last dose of vaccine should be given no less than 10 days before entering an endemic area.

The need for booster doses is not clear but the manufacturers recommend a booster at 4 years.
In view of the risk of adverse reactions, the manufacturers recommend that the patient should not leave the surgery building until 30 minutes after the injection.

• Mild adverse reactions are reported in as many as 20% of people.³ These include
  • Local pain and redness
  • Fever
  • Gastrointestinal symptoms
  • Headache, and myalgia
• The incidence of reactions usually decreases with each subsequent dose. In a study of over 14,000 US marines who received the vaccine, the rate of reactions for the 3 doses was around 16, 10 and 2 cases respectively per 10,000 doses.⁴
• Severe hypersensitivity, including angio-oedema or urticaria, occurs in 0.6% of patients.⁴
• 2.6 per 100,000 have a response severe enough to require admission to hospital.
• The hypersensitivity reaction may occur as late as 10 to14 days after the last dose. Hence, patients should have access to medical care for 10 days after the last dose.
• A history of allergies or urticaria may increase the risk for adverse reactions.⁴
• Adverse reactions tend to occur within 48 hours for the first dose but around 96 hours for the second.
• Cases of encephalitis and other potentially vaccine-related neurological symptoms have been reported. This association has not been definitively established and surveillance in the United States in the 1990s of more than 800,000 doses reported no neurological sequelae.

• The vaccine is recommended for people living in endemic and epidemic areas and for travellers planning extended trips to rural areas. This is usually defined as in excess of 30 days.
• For an area with active epidemic Japanese encephalitis, vaccination should be considered for shorter stays.
• Vaccination, even for short stays, is recommended if the person expects unprotected nocturnal outdoor exposure in an endemic area.

Risks and Benefits

The vaccine is highly effective but not without risks and the substantial risks of the disease and the risks of the vaccine have to be balanced,⁵ especially for stays of brief duration. As with malaria, prophylaxis must be supplemented by techniques to avoid being bitten by mosquitoes.