Some people will also experience inflammation of the brain, called encephalitis. About one of every four people who develop encephalitis will die. Of those who survive, about half will have a permanent disability, such as brain damage. JEV is the most common cause of encephalitis in the world.

Where do you catch JEV?

JEV infections occur in many areas throughout the world. Speak with a travel medicine clinician or consult the International Society of Travel Medicine (ISTM) resources webpage for more information.

How do you catch JEV?

JEV is transmitted by the bite of a mosquito. It does not spread from person to person.

Fortunately, the risk for JEV is low. In the 24-year period between 1993 and 2017, only 12 people traveling from the U.S. to the Far East got JEV. Most of those infected were members of the U.S. military who were there for duty.

How can you avoid JEV?

As with yellow fever and other illnesses spread by mosquitoes, the best way to avoid JEV is to avoid being bitten. Since mosquitoes most often feed between dusk and dawn, you and your family should take refuge indoors and remember to:

• Use mosquito netting over beds
• Wear long-sleeved shirts and long pants
• Wear insect-repellants with DEET on exposed skin
• Wear clothing treated with an insecticide such as permethrin
• Stay in accommodations with air conditioning or screens

The vaccine

How is the JEV vaccine made?

The only JEV vaccine currently available in the U.S. (called IXIARO) is approved for use in people 2 months of age and older. 

IXIARO was licensed in 2009 for adults and in 2013 for children 2 months of age and older. The vaccine is made by growing the virus in cells in the laboratory and then purifying it and killing it with a chemical. The vaccine cannot cause JEV because the virus is killed. 

The JEV vaccine is given in two doses with the second dose given 28 days after the first. Children younger than 2 years of age only get half of the regular dose. Adults 18–65 years old can get the second dose as early as 7 days after the first dose. The last dose should be given at least 1 week before travel. If you are recommended to get this vaccine before travel, you should schedule the vaccine at least five weeks prior to the trip because it takes about one month to get the two doses, and about another week to develop protective antibodies. Not all travelers need the JEV vaccine, so travelers should try to visit a travel clinic to determine their need based on their itinerary.

A booster dose is recommended after one year for people who remain at risk.

Does the JEV vaccine have side effects?

The JEV vaccine may cause mild side effects in a small number of recipients, including:

• Headache 
• Muscle aches
• Feeling “out of sorts,” called malaise
• Injection site reaction, like redness, swelling or a lump where the vaccine was given 

About 1 of 100 people will develop an injection-site reaction, and up to 20 of 100 will experience the other side effects listed. 

Who should get the JEV vaccine?

If you are staying in cities and your trip is shorter than one month, the risk of catching JEV is low. However, the JEV vaccine is recommended for travelers to high-risk areas during the JEV transmission season who are:

• Staying in rural or farm areas
• Staying for more than 30 days
• Planning activities such as biking, camping, or other outdoor activities with prolonged periods of exposure to mosquitoes
• Staying in accommodations without air conditioning, screens or bed nets

The vaccine should also be considered for those who are:

• Traveling to an area with an ongoing outbreak
• Traveling to an area known to have risk, but without specific information about destinations, activities or duration of travel

Other questions you might have

What should we do if our newly adopted daughter started the JEV vaccine while in China but did not get all doses before we brought her to the U.S.?

In the U.S., the Japanese Encephalitis (JEV) vaccine is only recommended for people who are likely to be exposed to JEV during travel. If your daughter will be making a trip back to China or to somewhere else that would increase her risk, then she would need to get the second dose. However, you should consult a travel clinic as the determination of need is based on trip-specific information, such as exactly where she is going, how long she will be there and what she will be doing.

Relative risks and benefits

Do the benefits of the JEV vaccine outweigh its risks?

JEV is a devastating illness that frequently causes permanent brain damage or death. Fortunately, JEV is an uncommon cause of disease in those traveling from the U.S. to regions of the Far East where the disease is prevalent. Less than one case per year occurs in U.S. travelers. When the disease does occur in travelers, it generally occurs only in those who have stayed for longer than 30 days, extensively camped or biked, or have stayed in rural areas. Therefore, the benefits of the JEV vaccine only outweigh its risks for those traveling to the Far East who are staying for at least one month and engaging in activities that increase their risk for exposure to infected mosquitoes.

Disease risks

• Fever, headache, stiff neck, nausea and vomiting
• Encephalitis (inflammation of the brain)
• Death following encephalitis (1 of every 4 people with encephalitis)
• Permanent disability, such as brain damage (1 of every 2 survivors who had encephalitis)

Vaccine risks

• Headaches, muscle aches, and malaise (up to 20 of every 100 people)
• Redness, swelling or lump at injection site (1 in 100 people)

References

Books

Orenstein WA, Offit PA, Edwards KM and Plotkin SA. Ebola Vaccines in Plotkin's Vaccines, 8th Edition. 2024, 577-607.

Websites

• International Society of Travel Medicine resources
• WHO information on Japanese encephalitis virus

Reviewed by Paul A. Offit, MD, on April 15, 2026