A Look at Each Vaccine: Japanese Encephalitis Virus (JEV) Vaccine

The mosquito-borne virus, Japanese encephalitis virus (JEV), does not just occur in Japan. JEV also occurs in many regions of the Far East, particularly in rural areas. However, the United States does not recommend the JEV vaccine for everyone traveling to the Far East. Rather, the vaccine is recommended based on aspects of travel such as location, length of stay, time of year, planned activities, and planned accommodations.

The disease

What is Japanese encephalitis virus?

Japanese encephalitis virus (JEV) is a virus that causes encephalitis (inflammation of the brain) and, like yellow fever virus, is transmitted by the bite of a mosquito. The disease is not spread from person to person. Most people infected with JEV virus don’t have any symptoms. If symptoms occur, they begin about one to two weeks after the bite. Afflicted individuals may have fever, headache, stiff neck, nausea, or vomiting. Some people will also experience swelling 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. Visit the Centers for Disease Control and Prevention (CDC) travel website for current information regarding the risk of getting JEV while traveling.

How do you catch JEV?

JEV is transmitted by the bite of a mosquito. Fortunately, the risk of catching JEV is very low. In the 24-year period between 1993 and 2017, only 12 people traveling from the United States to the Far East caught JEV. Most of those infected were members of the U.S. military who were in the field.

How can you avoid JEV?

As with yellow fever, the best way to avoid JEV is to avoid being bitten by a mosquito. Between dusk and dawn, when mosquitoes most often appear to feed, you and your family should take refuge indoors and remember to:

  • Use mosquito netting over the bed
  • Wear long-sleeved shirts and long pants
  • Wear insect-repellants with DEET on exposed skin
  • Wear clothing treated with insect-repellants 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 United States is licensed for use in people 2 months of age and older. Known as

IXIARO®, this vaccine 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 possibly cause JEV because the virus is killed. IXIARO® 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. Because it takes about one month to complete the vaccine series, and about another week to develop protective antibodies, you should plan ahead to be sure you have enough time to be protected before your trip. Decisions regarding use of this vaccine prior to travel are complex; therefore, travelers should seek more information by visiting the CDC’s travel website as well as by visiting a travel clinic to determine whether or not vaccine is needed.

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 headaches, muscle aches and malaise, and occur in about 1 or 2 of every 10 people who receive the JEV vaccine. Redness, swelling or a lump at the injection site occurs in about one in 100 vaccine recipients.

Who should get the JEV vaccine?

Fortunately, if you intend to stay in cities or stay for less than one month, the risk of catching JEV is very low. However, the JEV vaccine is recommended for those traveling 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 Japanese Encephalitis vaccine while in China but did not complete the series 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 in those infected. Fortunately, JEV is an uncommon cause of disease in those traveling from the United States to regions of the Far East where the disease is prevalent. Less than one case per year occurs in travelers from the United States to these regions. And 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 who are traveling to the Far East and who plan on staying for a long time and engaging in high-risk activities.

Disease risks

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

Vaccine risks

  • Headaches, muscle aches, and malaise (1 or 2 in every 10 people)
  • Redness, swelling or lump at injection site (1 in 100 people)



Plotkin SA, Orenstein W, Offit PA., and Edwards KM. Japanese encephalitis vaccines in Vaccines, 7th Edition. 2012, 511-548.


Centers for Disease Control and Prevention. Japanese Encephalitis. July 25, 2019.

Centers for Disease Control and Prevention. Chapter 4 Travel-Related Infectious Diseases. June 24, 2019.

Reviewed by Paul A. Offit, MD on May 06, 2020

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.