Japanese encephalitis vaccine: Ixiaro

The disease and why it matters

JE virus is a single-stranded RNA flavivirus closely related to dengue, West Nile and St. Louis encephalitis viruses, transmitted to humans by the bite of an infected mosquito (primarily Culex species); humans are incidental, dead-end hosts, and pigs and wading birds are the primary amplifying hosts. ^[1]

JE virus is the most common vaccine-preventable cause of encephalitis in Asia, occurring throughout most of Asia and parts of the western Pacific; transmission principally occurs in rural agricultural areas, often associated with rice cultivation and flood irrigation, and the area of transmission has expanded in Australia, with widespread transmission detected during 2021 to 2022. ^[1]

Most human infections are asymptomatic and fewer than 1% of infected people develop neurologic disease; the incubation period is 5 to 15 days; among patients with encephalitis the case-fatality rate is approximately 20% to 30% and 30% to 50% of survivors have serious neurologic, cognitive or psychiatric sequelae. There is no specific antiviral treatment. ^[1]

For most travelers to Asia the risk of JE is extremely low; the overall incidence among travelers from non-endemic countries is estimated at fewer than 1 case per 1 million travelers, but expatriates and travelers who stay for prolonged periods in rural areas with active transmission might be at similar risk to the susceptible pediatric resident population (6 to 11 cases per 100,000 children per year). Since 1993 only 16 JE cases among U.S. travelers have been reported to CDC (1993 to 2024). ^[1]

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Products available

One JE vaccine is licensed and available in the United States: IXIARO, an inactivated Vero cell culture-derived vaccine made by Valneva Austria GmbH, approved in March 2009 for people aged 17 years and older and in May 2013 for children aged 2 months through 16 years; other JE vaccines used in other countries are not licensed in the United States. ^[1]

The FDA IXIARO page is marked content current as of 11/15/2019; the label highlights list recent major changes dated 09/2018 for dosage and schedule. ^{[2] [3]}

Who should get it

The decision to vaccinate should be individualized and consider the specific itinerary, likelihood of future travel to JE-endemic countries, the high rate of death and disability when JE occurs, availability of an effective vaccine, the possibility but low probability of serious adverse events, and the traveler's perception and tolerance of risk; travel location, duration, activities, accommodations and seasonal patterns each influence risk, and JE virus transmission varies within countries and from year to year with often incomplete surveillance. ^[1]

Dosing and schedule, including catch-up

To administer a 0.25 mL dose, expel and discard half of the volume from the 0.5 mL prefilled syringe by pushing the plunger stopper up to the edge of the red line on the syringe barrel before injection; for all age groups the 2-dose series should be completed at least 1 week before potential exposure. ^{[3] [1]}

Limited data indicate that among adults who had previously received at least a primary series of mouse brain-derived inactivated JE vaccine, a single dose of IXIARO provided good protection through 12 to 23 months. ^[1]

Administration

• IXIARO is given by intramuscular injection only, as a single-dose 0.5 mL prefilled syringe (0.25 mL dose for children 2 months to under 3 years after discarding half the volume). ^[3]
• Question the recipient about reactions to previous vaccines and advise that IXIARO may not fully protect everyone and that personal precautions to reduce mosquito exposure (adequate clothing, repellents, mosquito nets) should be taken. ^[3]
• Travelers should avoid mosquito bites as the best way to prevent mosquito-borne diseases, including JE. ^[1]

Storage and handling

• Store in a refrigerator at 2 to 8 °C (35 to 46 °F); do not freeze; do not use after the expiration date; store in the original package to protect from light; a clear liquid with a white precipitate can be observed during storage. ^[3]

Contraindications and precautions

Safety and adverse events

Among adults the most common local reactions are pain and tenderness, reported in about one-third after the first or second dose, and the most common systemic reactions are headache (approximately 28%) and myalgia (approximately 16%); in children the most common systemic reaction is fever, with higher rates (more than 20%) in children aged 2 months to 2 years; serious adverse events are reported only rarely. ^[1]

Label rates: in infants 2 months to under 1 year, redness (more than 15%), fever (more than 20%), irritability (more than 15%) and diarrhea (more than 10%); in children 1 to under 3 years, fever (more than 20%); in children 3 to under 12 years, fever (more than 10%); in adolescents 12 to under 18 years, pain (15%) and tenderness (10%); in adults 18 years and older, pain (more than 25%), tenderness (more than 25%), headache (more than 20%) and myalgia (more than 10%). ^[3]

Special populations

Pregnancy: no studies of IXIARO in pregnant women have been conducted; pregnancy is a precaution and in most instances vaccination should be deferred, but pregnant women who cannot delay travel should be vaccinated when the risk for JE virus infection outweighs the theoretical risk from immunization; the label says human data from trials and post-marketing experience are insufficient to establish the presence or absence of drug-associated risk, and the VIS says it is believed that infection in a pregnant woman could harm her unborn baby. ^{[1] [3] [4]}

Lactation and age: it is not known if IXIARO is excreted in human milk; safety and effectiveness have not been established in infants younger than 2 months, and clinical studies did not include sufficient numbers of subjects aged 65 years and older to determine whether they respond differently. ^[3]

Counseling pearls for the pharmacy counter

• JE risk is very low for most travelers but higher for people living in or traveling for long periods to areas where the disease is common; most infected people have no symptoms, but about 1 person in 4 with encephalitis dies and up to half of survivors have permanent disability. ^[4]
• Vaccination does not replace mosquito bite precautions: use adequate clothing, repellents and mosquito nets. ^[3]
• Complete the 2-dose series at least 1 week before potential exposure; a booster is considered at 1 year or more if exposure continues. ^{[3] [1]}

Common patient questions

References

1. Centers for Disease Control and Prevention. Japanese encephalitis. In: CDC Yellow Book: Health Information for International Travel, 2026 edition. Page dated February 20, 2026. Accessed October 12, 2026. https://www.cdc.gov/yellow-book/hcp/travel-associated-infections-diseases/japanese-encephalitis.html. Accessed October 12, 2026.
2. U.S. Food and Drug Administration. IXIARO product page (content current as of 11/15/2019). Accessed October 12, 2026. https://www.fda.gov/vaccines-blood-biologics/vaccines/ixiaro. Accessed October 12, 2026.
3. IXIARO (Japanese encephalitis vaccine, inactivated, adsorbed), suspension for intramuscular injection: prescribing information. Valneva Austria GmbH. U.S. Food and Drug Administration. Accessed October 12, 2026. https://www.fda.gov/media/75777/download?attachment. Accessed October 12, 2026.
4. Centers for Disease Control and Prevention. Japanese encephalitis vaccine: what you need to know. Vaccine Information Statement, August 15, 2019. Accessed October 12, 2026. https://www.cdc.gov/vaccines/hcp/current-vis/japanese-encephalitis.html. Accessed October 12, 2026.