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Japanese encephalitis

Japanese encephalitis virus is the leading cause of vaccine-preventable encephalitis in Asia and the western Pacific. For most travelers to Asia, the risk for Japanese encephalitis is very low but varies based on destination, length of travel, season, and activities. Most people infected with Japanese encephalitis do not have symptoms or have only mild symptoms. However, a small percentage of infected people develop inflammation of the brain (encephalitis), with symptoms including sudden onset of headache, high fever, disorientation, coma, tremors, and convulsions. About 1 in 4 cases are fatal. To prevent getting sick from Japanese encephalitis, use an EPA-registered insect repellent, wear long-sleeved shirts and long pants, and get vaccinated.


The most effective way to prevent infection from Japanese encephalitis virus is to prevent mosquito bites. Mosquitoes bite during the day and night. Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and get vaccinated before traveling, if vaccination is recommended for you.


Inactivated Vero cell culture-derived Japanese encephalitis vaccine (manufactured as IXIARO) is the only Japanese encephalitis vaccine licensed and available in the United States. This vaccine was approved in March 2009 for use in people aged 17 years and older and in May 2013 for use in children 2 months through 16 years of age. Other Japanese encephalitis vaccines are manufactured and used in other countries but are not licensed for use in the United States.

IXIARO is given as a two-dose series, with the doses spaced 28 days apart. Adults aged 18–65 years 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. A booster dose (third dose) should be given if a person has received the two-dose primary vaccination series one year or more previously and there is a continued risk for Japanese encephalitis virus infection or potential for reexposure.

For adults and children aged 3 years or older, each dose of IXIARO is 0.5 mL. For children aged 2 months through 2 years, each dose is 0.25 mL.

More information about Japanese encephalitis vaccine can be found in the Vaccine Information Statement for IXIARO Japanese Encephalitis Vaccine Cdc and in the IXIARO product information available at the FDA IXIARO webpage.

Japanese encephalitis vaccine for travelers. Japanese encephalitis is a very low-risk disease for most travelers to Japanese encephalitis-endemic countries. However, some travelers will be at increased risk of infection based on factors including longer periods of travel, travel during the Japanese encephalitis virus transmission season, spending time in rural areas, participating in a lot of outdoor activities, and staying in accommodations without air conditioning, screens, or bed nets.

All travelers to Japanese encephalitis-endemic countries should take steps to avoid mosquito bites, and discuss the need for vaccination with their healthcare provider. The discussion should include the risks related to the specific travel itinerary, the likelihood of future travel to countries where Japanese encephalitis occurs, the possible severe outcomes of Japanese encephalitis disease, and information about the vaccine including cost and possible side effects.

Japanese encephalitis vaccine recommendations. Japanese encephalitis vaccine is recommended for persons moving to a Japanese encephalitis-endemic country to live, longer-term (eg, 1 month or longer) travelers, and frequent travelers to Japanese encephalitis-endemic areas. Japanese encephalitis vaccine also should be considered for shorter-term (eg, less than 1 month) travelers with an increased risk of Japanese encephalitis based on planned travel duration, season, location, activities, and accommodations. Vaccination also should be considered for travelers to endemic areas who are uncertain of specific duration of travel, destinations, or activities.

Japanese encephalitis vaccine is not recommended for travelers with very low-risk itineraries, such as shorter-term travel limited to urban areas or travel that occurs outside of a well-defined Japanese encephalitis virus transmission season.

Precautions and contraindications. A serious allergic reaction after a previous dose of IXIARO, after any other Japanese encephalitis vaccine, or after any component of IXIARO is a contraindication to further doses. The vaccine contains protamine sulfate, a compound known to cause allergic reactions in some people.

No studies of IXIARO in pregnant women have been conducted. Therefore, administration of IXIARO to pregnant women usually should be deferred. However, pregnant women who must travel to an area where risk for infection is high should be vaccinated when the theoretical risk of immunization is outweighed by the risk of infection.

Reactions to Japanese encephalitis vaccine. Reactions to IXIARO are generally mild and include pain and tenderness, mild headaches, myalgia (muscle aches), and low-grade fevers.

Transmission of Japanese Encephalitis Virus

Japanese encephalitis virus, a flavivirus, is closely related to West Nile and St. Louis encephalitis viruses. Japanese encephalitis virus is transmitted to humans through the bite of infected Culex species mosquitoes, particularly Culex tritaeniorhynchus.

The virus is maintained in a cycle between mosquitoes and vertebrate hosts, primarily pigs and wading birds. Humans are incidental or dead-end hosts, because they usually do not develop high enough concentrations of Japanese encephalitis virus in their bloodstreams to infect feeding mosquitoes.

Japanese encephalitis virus transmission occurs primarily in rural agricultural areas, often associated with rice production and flooding irrigation. In some areas of Asia, these conditions can occur near urban centers.

In temperate areas of Asia, Japanese encephalitis virus transmission is seasonal. Human disease usually peaks in the summer and fall. In the subtropics and tropics, transmission can occur year-round, often with a peak during the rainy season.


  • Less than 1% of people infected with Japanese encephalitis (JE) virus develop clinical illness.
  • In persons who develop symptoms, the incubation period (time from infection until illness) is typically 5-15 days.
  • Initial symptoms often include fever, headache, and vomiting.
  • Mental status changes, neurologic symptoms, weakness, and movement disorders might develop over the next few days.
  • Seizures are common, especially among children.


  • No specific treatments have been found to benefit patients with JE, but hospitalization for supportive care and close observation is generally required.
  • Treatment is symptomatic. Rest, fluids, and use of pain relievers and medication to reduce fever may relieve some symptoms.


  • Among patients who develop encephalitis, 20% – 30% die.
  • Although some symptoms improve after the acute illness, 30%-50% of survivors continue to have neurologic, cognitive, or psychiatric symptoms.

This information was developed by the Centers for Disease Control and Prevention. Japanese Encephalitis Information Page. Available at: Accessed July 17, 2020.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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