Japanese Encephalitis Vaccine
A summary Paper from the World Health Organization

Background
Japanese encephalitis (JE) is an acute viral infection of the central nervous system occurring in a large number of countries/areas of Asia, including Cambodia, China, Indonesia, Japan, Lao People's Democratic Republic, Malaysia, Myanmar, the Philippines, Republic of Korea, Thailand, Vietnam, south-eastern Russian Federation and the Indian subcontinent. In recent decades, JE has gradually spread to previously non-affected Asian regions, and a small outbreak was recently reported from islands in the Torres strait off the Australian mainland.

The annual incidence of clinical infection in endemic areas ranges from 10-100 per 100 000 population. Close to 3 billion people are now living in JE endemic regions, where more than 70 million children are born each year.

Bloodsucking Culicine mosquitoes (mainly Culex tritaeniorhynchus, Cx.gelidus and Cx.fuscocephala) transfer the virus to humans from infected animals, in most cases domestic pigs and wading birds. Human beings are not considered a reservoir for viral transmission. The virus infection rate in the mosquito vector ranges from <1% to 3%. Culex breeds preferably in water pools and flooded rice fields, and most of the human cases occur in rural areas. However, outbreaks have occurred also in periurban and urban populations of several major Asian cities. In most regions the period of transmission starts in April or May, and lasts until September or October. In some tropical and subtropical areas the incidence peaks during and shortly after the rainy season, the timing depending on the region. However, where irrigation permits mosquito breeding throughout the year, transmission may occur even in the dry season.

Serological surveys shows that most of the people living in JE endemic areas are infected with the virus before the age of 15. Usually, however, the infection does not cause clinical symptoms; an estimated average of 1 in 300 JE viral infections results in symptomatic illness. In hyperendemic areas, half the number of cases occur before the age of 4 years, and almost all before the age of 10. In countries such as Japan and the Republic of Korea, and in some regions in China, the incidence of JE has decreased over several decades, in part due to extensive use of JE vaccines, but also as a consequence of improving socioeconomic conditions and changing agricultural practices. In some of those regions a shift is also observed in the age distribution of cases towards older children and adults. In Japan, the age specific incidence has become bimodal, peaking in young children and the elderly. The disease is uncommon among short term visitors and tourists to endemic areas.

Clinical JE follows an incubation period of 4-14 days and is mostly characterised by sudden onset of fever, chills and aches, including headaches and sometimes meningismus, particularly in adults. In children, gastrointestinal pain and dysfunction may dominate the initial stage of the disease. Convulsions are also very common in paediatric patients. Although JE is often a mild disease, leading to an uneventful recovery, some cases rapidly progress to severe encephalitis with mental disturbances, general or focal motor abnormalities, and progressive coma. Of the approximately 50 000 cases of JE officially reported each year, about 10 000 end fatally and a very high percentage of the survivors are left with neurological and psychiatric sequelae, requiring extensive care. Most fatalities and residual sequelae occur in children aged <10.

Since most infections occur in childhood, experience with JE disease in pregnant women is limited. However, studies from Uttar Pradesh (India) indicate a high risk of abortion among those infected during the first timesters. The potential impact of concurrent infections (in particular HIV) on the outcome of JE virus infection is not yet established. Also, several aspects of JE epidemiology require further studies. Example of problems in need of further investigation include the reasons for the geographical distribution and the mechanisms of viral persistence between epidemics.

The pathogen
Japanese encephalitis virus (JEV) belong to the flavivirus family. Flaviviruses have single stranded RNA, and are spherical, enveloped and about 40 mm in diameter, and usually vector borne. JEV is antigenically related to several other flaviviruses, including St Louis encephalitis virus and West Nile virus. Virus specific as well as cross reactive neutralising epitopes for JE have been identified on the envelope glycoprotein. A number of antigenic subgroups of the virus are known, although so far, possible differences between these subgroups in terms of virulence or host preferences are poorly defined. Following an infectious mosquito bite, the initial viral replication occurs in local and regional lymph nodes. Viral invasion of the central nervous system probably occurs via the blood. Certain nerurotransmitter receptors are believed to be involved in the binding of JE virions to cells in the central nervous system. The affinity of JE virus for neural tissue leads to propagation in the brain.

Etiological diagnosis of JE is mainly based on serological testing, using IgM-capture ELISA, that detects specific IgM in the cerebrospinal fluid or in the blood of almost all patients within 4-7 days of onset of disease. Other diagnostic methods include recently developed dot-blot or immunoprecipitation IgM assays, suitable for use in the field, and traditional tests that monitor significant changes of JE specific antibody titres in sequential serum samples. The virus may be recovered in various cell cultures inoculated with blood collected during the early stages of the disease, or from the cerebrospinal fluids (or brain) in advanced cases of encephalitis. Polymerase chain reaction tests for the detection of virus specific genomic material particularly in cerebrospinal fluid, have also been developed.

Protective immune response
Protection is associated with the development of neutralising antibodies. Although no internal standard has yet been established, neutralising antibody titres of 1:10 or more are commonly accepted as evidence of protection. A role for cell mediated immune mechanisms in protection against JE virus has been demonstrated in experimental studies on mice.

The justification for vaccine
JE is the leading cause of viral encephalitis in Asia. In endemic areas, annual incidence of clinical disease ranges from 10-100 per 1,000,000 population. Case fatality averages 30% and a very high percentage of the survivors are left with permanent neuropsychiatric sequelae. There is no efficient drug treatment for this disease. In recent decades the JE virus has caused epidemics in previously unaffected countries, including India, Myanmar, Nepal, Sri Lanka, Thailand and Vietnam. No effective method of environmental control of JE transmission is known. Although socioeconomic improvements and changes in agricultural practices are likely to reduce viral transmission in some places, large scale vaccination of affected populations with effective and affordable vaccines appears to be the logical control measure, at least in the short term. The impact of large scale JE vaccination is clearly documented in some regions of China, and the systematic vaccination has also probably contributed significantly to the declining incidence rates in Japan, Republic of Korea and Thailand.

Vaccines against Japanese encephalitis
Three types of JE vaccine are currently in large scale production and use, namely: mouse brain derived inactivated vaccine; cell culture derived inactivated vaccine; and cell culture derived live attenuated vaccine.

Mouse brain derived inactivated vaccine
The mouse brain derived inactivated JE vaccine is produced in several Asian countries. So far, this is the only type of JE vaccine that is commercially available on the international market. Crude versions of such vaccines were produced as early as the 1930's. However, variable immunogenicity and fear of vaccine induced encephalitis owing to their high content of myelin basic protein prompted the development of the current vaccine, which is produced to secure consistent immunogenicity and which has an extremely low content of myelin basic protein (<2 ng per ml). The commercially available mouse brain derived JE vaccine is based either on the Nakayama strain, which was isolated in Japan in 1935, or on the Beijing-1 strain. This vaccine has proved to be efficacious even in Thailand, where different topotypes of the JE virus are prevalent. On the other hand, cross immunisation studies in mice comparing the Nakayama strain with JE virus strains from different Asian regions have shown that the so called Beijing-1 strain induces a broader neutralising antibody response. For this reason, and because of the higher antigen yield in the mouse brain following inoculation of the Beijing strain, the Nakayama strain has been replaced in several mouse brain derived JE vaccines.

The mouse brain derived JE vaccine is given subcutaneously in doses of 0.5 ml or 1 ml, the lower dose being for children aged 1-3. Due to likely interference with remaining maternal antibodies, children are usually not vaccinated before the age of 1 year. The manufacturers of the internationally marketed vaccine recommend that primary childhood immunisation involve 2 injections at an interval of 1-2 weeks. In several Asian trials, primary immunisation has a disease preventing efficacy of >95%; 91% efficacy was achieved in a placebo controlled trial. The seroconversion rates appear not to be reduced when other childhood vaccines are given simultaneously.. However, the primary vaccination schedules vary considerably among different Asian countries. Furthermore, the optimal intervals and number of booster doses are poorly defined and may show considerable local variation reflecting, for example, a different boostering impact of prevailing cross reacting flaviviruses. Many Asian countries have adopted, a schedule of 2 primary doses approximately 4 weeks apart, followed by a booster after 2 year, with subsequent boosters at 3 year intervals. However, the duration of immunity after serial booster doses has not been well established.

Immunogenicity studies conducted in Western countries, where interference by other flaviviruses is unlikely, has shown that seroconversion was obtained only in about 80% of the vaccinees following the primary 2 dose schedule. Also, in 90% of the vaccinees the titre of neutralising antibody declined within 6-12 months to levels below the established protective titre level. However, in United States soldiers, a 3 dose schedule based on vaccination on days 0, 7 and 30 resulted in 100% seroconversion, significantly higher titres of neutralising antibodies and persistence of those high levels for at least 3 years.

In its lyophilised version, this vaccine is stable at 4⁰C for at least 1 year; following reconstitution it retains its original potency for at least 2 weeks at 22⁰C, whilst at 37⁰C, potency is still 85% after 2 weeks. This stability obviously facilitates its use under field conditions in hot climates. In endemic regions of Thailand, JE vaccination has been successfully incorporated into the national immunisation programme. However, owing to its relatively high price, large scale vaccination with the mouse brain derived JE vaccine is unlikely to be affordable in the poorest endemic countries of Asia.

Vaccine associated adverse effects
In general the mouse brain derived JE vaccine has been considered relatively safe, although local reactions such as tenderness, redness and swelling occur in about 20% of the vaccinees. A similar percentage may experience mild systemic symptoms, including headache, myalgia, gastro intestinal symptoms and fever. Vaccine related neurological complications were not observed in Japanese studies from 1955-1966. However, in recent years, several cases of acute encephalitis temporally linked to JE vaccination have been reported. From the Republic of Korea, 3 such cases were recently reported, of which 2 were fatal. Furthermore, hypersensitivity reactions, in some cases serious generalised urticaria, facial angioedema or respiratory distress, have recently been observed in adult Western vaccine recipients. In 1994, 2 cases of fatal anaphylaxis attributed to JE vaccinations were observed in the Republic of Korea. In a prospective study among United states military personnel in Okinawa (Japan), the overall allergic reactions were more prone to develop hypersensitivity to the vaccine. The vaccine components responsible for these adverse effects have not been identified, although allergic reactions to gelatin which is used to stabilise the vaccine is suspected in some cases. Such reactions may occur as late as 12-72 hours following immunisation.

Except for a history of hypersensitivity to the vaccine there are no contraindications to JE vaccination.

Cell culture-derived inactivated vaccine
This vaccine is manufactured in China and based upon the Beijing P-3 strain of JE virus which provide broad heterologous immunity in mice, and high viral yields when propagated in primary hamster kidney cells. Primary immunisation with infants with this formalin-inactivated vaccine results in about 85% protection. The vaccine is inexpensive, and 90 million doses are distributed for internal Chinese use every year. However, in China this vaccine will gradually be replaced by the cell culture derived live attenuated vaccine.

Cell culture-derived live attenuated vaccine
This Chinese vaccine is based on a stable neuro-attenuated strain of the JE virus (SA-14-14-2). In non-endemic areas, 1 single dose of this vaccine induced an antibody response in 83%-100% showed a serological response. Side effects are reported to be minimal. The cost per dose in China is very low. Currently, 40 million doses of this vaccine are produced annually for use in China.

General WHO position on new vaccines
Vaccines for large scale public health use should:

• meet the quality requirements as defined in the GPV policy statement on vaccine quality
• be safe and have a significant impact against the actual disease in all target populations
• if intended for infants or young children, be easily adapted to schedules and timing of the national childhood immunisation programmes
• not interfere significantly with the immune response to other vaccines given simultaneously
• be formulated to meet common technical limitations, eg in terms of refrigeration and storage capacity
• be appropriately priced for different markets.

WHO position of Japanese encephalitis vaccine
Only a freeze dried, mouse brain derived vaccine based on either the Nakayama or Beijing strain of the JE virus is commercially available internationally. This vaccine is manufactured according to current international quality requirements, and is in general considered efficient and safe for immunisation of children. At least in Thailand, the JE vaccine has been successfully incorporated in the national immunisation programme, and there is no evidence of interference with simultaneously given vaccines. Hence, where affordable, this vaccine is recommended for use in JE endemic regions.

Rare, but serious, neurological side effects attributed to this JE vaccine have been reported for endemic as well as non-endemic regions. Also, about 0.6% of Western adults, who in recent years were vaccinated prior to visits to endemic areas, have experienced allergic reactions to components of this vaccine. Therefore, an increased awareness of neurological as well as allergic adverse effects is necessary in endemic areas, in particular in countries where changing epidemiological patterns motivate JE vaccination of adults as well as children. Phase IV studies, that include the required system for surveillance and reporting of rare events, should be performed in children. The possibly higher incidence of adverse reactions in adults may be addressed in appropriately powered placebo controlled trails.

Although several of the JE endemic Asian countries produce the mouse brain derived vaccine for their national consumption, there is a problem of vaccine supply in many endemic areas. Unfortunately, the price of the commercially available vaccine is an obstacle to its use in many of the poorest countries in the region.

Although, both genotypic and phenotypic variations of JE viruses have been demonstrated, there is little evidence to suggest that the Nakayama vaccine strain results in reduced protection against infection by local virus strains.

Further information is needed concerning the duration of protection induced by JE vaccination. The practice of repeated immunisation with JE vaccine requires careful review. It is not known whether exposure of vaccinated individuals to natural infection contributes to protection. The possible interaction between JE virus infection and other flaviviruses could have an impact both on the result of primary immunisation and on the requirement for boosters.

Effective and inexpensive JE vaccines are needed in large quantities in order to meet WHO and national public health objectives.

Reproduced from and with acknowledgement to the Weekly Epidemiological Record 1998, 73, 337-344.