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Disease information 

Virus or bacteria

Japanese encephalitis virus (JEV) is a virus that causes encephalitis (inflammation of the brain). Despite its name, the virus does not occur only in Japan. People in 24 countries throughout South-East Asia and the Western Pacific are at risk for infection.

How it spreads

JEV spreads by the bite of a mosquito carrying the virus; it does not spread from person to person.

JEV is not contagious. Someone with JEV cannot infect another person.

Symptoms

Early onset symptoms typically include:

• Headache
• Fever
• Cough
• Upper respiratory symptoms
• Nausea
• Vomiting
• Diarrhea

If the virus infects the nervous system, additional symptoms may include:

• Stiffness of muscles
• Seizures
• Sensitivity to light
• Altered mental status
• Involuntary movements
• Paralysis
• Swelling of the brain
• Coma
• Death

Long-term or permanent disabilities can include:

• Memory loss
• Impaired comprehension
• Seizures
• Muscle weakness or paralysis
• Impaired coordination

Typical disease timeline

JEV causes early onset symptoms in the first 1 to 2 weeks of infection. During the second week, the virus may affect the nervous system leading to additional symptoms and complications. People who survive after having complications of JEV often experience long-term or permanent disabilities.

Possible complications

• Permanent brain damage (50 of every 100 with encephalitis)
• Coma
• Death (25 of every 100 with encephalitis)

Other things to know

• JEV can also infect pigs, birds, and horses, and these animals contribute to its spread.

Vaccine information

The Japanese encephalitis virus vaccine is used to prevent Japanese encephalitis virus (JEV).

Vaccine type

• Killed viral vaccines

Vaccine side effects

Common

• Headaches (20 of 100 recipients)
• Muscle aches (20 of 100 recipients)
• Feeling "out of sorts" (20 of 100 recipients)
• Redness, swelling or a lump at the injection site (1 of 100 recipients)

Rare

• None

Very rare

• Hypersensitivity (22 to 44 of 1,000,000 recipients)
• Neurologic reactions, like seizures or brain swelling, (less than 11 to 22 of 1,000,000 recipients)
• Serious allergic reaction (less than 1 of 1,000,000 recipients)

Other things to know

• Not all travelers need this vaccine. A travel medicine expert or public health official can help determine which travelers should consider vaccination. 
• This vaccine is given in two doses separated by 28 days for most people. It takes an additional week for immunity to develop. So, it is important to allow time for vaccination before travel.
• Additional doses of vaccine should not be given if an allergic reaction occurs after the first dose.

Reasons to get the JEV vaccine

• While JEV can be mild, some people will experience swelling of the brain (encephalitis). This can result in permanent brain damage (30-50 of every 100 with encephalitis). Sadly, some people will become comatose and die (20-30 of every 100 with encephalitis).
• The JEV vaccine is safe. 

When may this vaccine be necessary

• One JEV vaccine is currently available in the U.S. It can be given to those 2 months of age or older.
• The JEV vaccine is most often considered a travel vaccine in the U.S., but not all travelers are recommended to receive it.
• Travelers to South-East Asia and the Western Pacific regions may be recommended to get this vaccine.
• Please consult with a travel medicine expert before traveling.