Matthew C. Mori

Dr. Robert Siegel

HBIO 115B: The Vaccine Revolution

June 3, 2000


Live attenuated Japanese encephalitis (SA14-14-2) vaccine:

The best alternative?


Japanese encephalitis (JE) causes over 15,000 deaths a year of children in southeast Asia. Three vaccines are licensed against the Japanese encephalitis virus. The inactivated mouse-brain derived JE vaccine is available on the international market. Primary hamster kidney (PHK) cell derived inactivated and live attenuated JE vaccines are widely used in China. A comparison between the inactivated mouse-brain derived vaccine and the PHK-derived live attenuated vaccine showed similar immunogenicity (84-100% seroconversion after 3 doses vs. 94-100% seroconversion after 2 doses), efficacy (91% with two doses vs. 97% with two doses), and incidence of minor side effects. The inactivated JE vaccine was associated with rare incidence of severe allergic reactions and neurological complications. The live attenuated vaccine is a better JE vaccine option for countries endemic for JE because of its shorter dosage schedule and low cost.


Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis in the world. The virus causes acute nervous system infections which lead to over 50,000 cases of encephalitis and 15,000 deaths yearly. Thirty percent of survivors develop serious permanent neurological sequelae (Solomon, 2000). Although most endemic regions are in southeast Asia, travelers from all countries must take protective measures. The virus continues to spread to new countries. In 1998, two Japanese encephalitis (JE) cases occurred in Australia (Solomon, 2000). In endemic regions, JE is a childhood disease with most infections occurring before age 15 (Solomon, 2000). Encephalitis develops in 1 to 20 of 1,000 cases (ACIP, 1993). The mosquito, Culex tritaeniorhynchus, transmits the virus to humans and amplifying hosts, such as swine, birds, and other vertebrates. Due to the mode of transmission, JE is a seasonal disease mainly occurring between May and September in rural areas.

The use of vaccines and mosquito control has helped lead to a decrease in incidence of JE (Sohn, 2000). Currently, there are three licensed JE vaccines. A formalin-inactivated JE vaccine is used worldwide. The Research Institute of Microbial Diseases (Biken) at Osaka University in Japan is one of the main manufacturers of this vaccine. Although the vaccine has been licensed in Japan since 1954, the vaccine was just licensed in the US in 1992 (ACIP, 1993). This mouse brain derived vaccine is prepared using the Nakayama-NIH strain (other manufacturers use the Beijing-1 strain). The inactivated vaccine is the only JE vaccine available on the international market. Two other JE vaccines are licensed in China. An inactivated JE vaccine derived from primary hamster kidney (PHK) cells is often used. In the past ten years, a live, attenuated vaccine derived from PHK cells and produced from the SA14-14-2 viral strain has begun to replace the inactivated vaccine because of its low cost (Hennessy et al., 1996). This vaccine, licensed in 1988 in China, has been distributed to over 100 million children (Tsai, 1994).

The Chinese live, attenuated vaccine is a better alternative for widespread use than the currently used inactivated Biken vaccine because of its safety record and lower cost. This paper will compare and examine dosage schedules, immunogenicity, efficacy, safety, and cost for the two vaccines.


Dosage schedules differ for endemic and non-endemic regions. Children in endemic areas may need fewer doses because of previous immunity or subsequent exposures to flaviviruses that boost antibody levels (ACIP, 1993). In endemic areas, the inactivated Biken vaccine is administered subcutaneously in two 0.5-mL doses one to four weeks apart. Primary vaccination varies from the age of 18 months in Thailand to 3 years in Japan. Boosters often are given one year after primary vaccination and at intervals of 3 years thereafter (Tsai, 1994). In non-endemic regions, the Advisory Committee on Immunization Practices (ACIP) recommends a three-dose regimen on days 0, 7, and 30 with a booster given at one year and every three years thereafter (ACIP, 1993). A recent study of US soldiers found that a three-dose regimen on days 0,7, and 14 was also sufficiently immunogenic (Defraites et al., 1999). Normal administration in China consists of a 0.5-mL dose at years 1 and 2. A final booster is given in year six (Hennessy et al., 1996).


Japanese encephalitis immunogenicity studies define protection and seroconversion as neutralizing antibody titers greater or equal to 1:10 (Tsai, 1994). Inactivated JE vaccine showed high seroconversion rates after 2 doses in endemic areas and after 3 doses in non-endemic areas. Live attenuated vaccine showed high seroconversion rates after 1 or 2 doses in endemic areas.

Inactivated vaccine

Several Asian immunogenicity studies have found greater than 95% seroconversion for the inactivated vaccine in endemic areas (Tsai, 1994). A study in the non-endemic Kasauli, India of 42 male volunteers with no neutralizing antibody before vaccination showed all subjects with protective levels by day 35 after the second dose. Sixteen other seropositive male subjects were given boosters. All 16 showed a significant increase in antibody levels. The researchers concluded that two doses could induce 100% seroconversion. This rate of seroconversion is comparable to seroconversion rates in previous mass field studies in Japan, Taiwan, and Thailand (80-100% seroconversion) (Rao Bhau et al., 1988). However a longer study period would have made this study much stronger. The antibody levels are protective at 35 days, but how useful is it if antibody levels are no longer protective in six months? The researchers recognized this and suggested a follow-up booster after one year. In addition, a control of other seronegative men would have also made sure that seroconversion was not caused by subclinical infection. The study is limited to male adult volunteers. Rates cannot be generalized to children. Nevertheless, the 100% rate of seroconversion, in conjunction with previous field studies showing high rates of seroconversion in children, provide compelling evidence that the inactivated vaccine produces sufficient short-term levels of protective antibody after two or three doses.

Several other studies of US and British military populations and US inhabitants show that seroconversion is much higher after 3 doses than after 2 doses. One study of 1152 subjects in Nepal, tested 75 subjects for immune response. Of the 47 seronegative subjects given 3 doses, 42 (89%) seroconverted. Of the 14 seronegative subjects given 2 doses, 5 seroconverted (35%). Although this is a distinct difference (the study did not give statistical analysis), the study groups consisted of different populations. The 3-dose study group consisted of American soldiers and Gurkha soldiers while the 2-dose study group consisted of Nepalese male civilians (Henderson, 1984). The demographics were not given, so the groups could be significantly different. Other confounds or small sample size could also explain the difference.

A US study of 118 Americans given a two-dose series and 72 Americans given a three-dose series provided more concrete evidence to support the three-dose schedule. The seroconversion rate was 77% for the two dose series and 99% for the three dose series. Confidence intervals would have helped with assessing the significance of those numbers. After six months, only 10 of 24 two-dose recipients had lost their protective levels. After a booster, all 24 once again had protective neutralizing antibody levels (Poland et al., 1990). Unfortunately, the study did not address the long-term protectiveness of the 3-dose regimen. Another study of American soldiers showed that a two-dose primary immunization with a 6-month booster gave a 100% seroconversion rate (n=25). Before the booster, 18 of 27 subjects did not have protective antibody levels (Sanchez et al., 1990). Based on these studies, the three-dose primary immunization regimen is the most effective for people from non-endemic areas. Boosters are necessary for long-term stay.

Live attenuated vaccine

Several small Chinese studies have shown 85% to 100% seroconversion after one dose. After 12 months only 31% remained seroconverted (Tsai, 1994). A Korean study of 68 seronegative children ages one to three years showed high short-term seroconversion rates (96%) at four weeks after one dose of the vaccine. Sixteen seropositive children also showed significant increases in geometric mean titer (GMT) (Sohn et al., 1999). Long term follow-up and a non-vaccinated control would have strengthened the study.

Another study in China tested between a two-dose one-month immunization schedule and three-month immunization schedule. The study was limited because it used 231 children ages 12 to 15 years. Two different vaccine production lots with varied infectious titer were used. Administration of the two lots resulted in different seroconversion rates after one dose, but similar seroconversion rates after two doses. After the first dose, seroconversion rates varied from 72% (n=53) to 100% (n=56). After the second dose, seroconversion was 100% in both one-month groups and 94% and 100% in the two three-month groups. GMTs were much higher in the three-month group (Tsai et al., 1998). Reporting more data points for seroconversion and GMTs at various time intervals would have given a more complete picture of the immune response. Again, long-term testing would have been useful, but this study gives an idea of seroconversion rates in non-endemic areas. Vaccination with one dose could be protective, but vaccination with two doses gave close to 100% seroconversion rates.


Both vaccines have been found to have 90 to 97% efficacy rates.

Inactivated vaccine

In 1965, initial studies in Taiwan showed that two doses had 80% efficacy in protecting patients the first year after vaccination (Tsai, 1994). A placebo-controlled, blinded, randomized study performed in Thailand from November 1984 to December 1986 showed 91% efficacy (95% CI – 70 to 97 percent) for 2 doses of vaccine. The researchers split 65,224 children ages 1 to 14 years into a monovalent (Nakayama strain) JE vaccine group, a bivalent (Nakayama and Beijing-1 strains), and a placebo (tetanus toxoid) group. They administered two doses at a seven-day interval. Eleven cases of JE occurred in the placebo group (n=21,516, 51 per 100,000) and one case of JE occurred in each of the vaccine groups (n=21,628, n=22,080, 5 per 100,000 for each) (Hoke et al., 1988). The strengths of this study include a placebo control, two years of observation, and huge sample size. The researchers took special measures to find mild or not hospitalized cases by conducting a post study census and serological testing follow-up. But there should have been a general population control to see what the baseline JE rate was without intervention.

Live attenuated vaccine

Trials in Chinese children with a two dose administration a year apart yielded protection rates from 98 to 100 percent with high confidence intervals (Tsai, 1994). A retrospective study was done on all cases of encephalitis from 1993 in Sichuan Province, China. 1299 matched controls of similar age (mean age: 4.7 years) were found from the villages of the 56 cases. The researchers obtained vaccine histories and compared the two groups. Of the 56 cases, 38 had no previous vaccination, 11 had one dose vaccination, 6 had two dose vaccination, and 1 had three dose vaccination. Administration of one dose was found to have an 80% efficacy (95% CI – 44 to 93%) and administration of two doses were found to have an efficacy of 97.5% (95% CI – 86 to 99.6%) (Hennessy et al., 1996). This study depended on unreliable vaccine histories and did not mention when the vaccinations were given. Time since last vaccination would be very important in determining the effectiveness of the vaccine. Ideally, the researchers would have done a prospective study with appropriate controls, but the ethical obligations of treatment make this impossible. One solution is to maintain surveillance of a large population and compare the JE incidence rate with traditional incidence rates.


Severe allergic reactions and neurological complications have been associated with the inactivated vaccine. Both the inactivated and live attenuated vaccine are associated with mild symptoms.

Inactivated vaccine

Since 1989, several reports of adverse reactions associated with JE vaccination have emerged in Australia, Canada, and Denmark (Nazareth et al., 1994). A letter to the editor in the October 5, 1991 The Lancet describes three cases of severe adverse events following JE vaccination in Australia. Severe rashes and swelling occurred within three days of the second dose of vaccination. No other medications or vaccinations were given at the same time. The vaccines came from the same batch. Twenty-seven other mild adverse events were reported in 1990 and 1991. Because there is no active surveillance in Australia, the actual numbers may be higher than reported (Ruff et al., 1991). Another report from Denmark describes the increase in incidence of urticaria, angioedema, and other allergic reactions after the first reported incidence in 1989. High rates of adverse reactions were associated with specific batches (up to 15 per 10,000 vaccinations). 1992 and 1993 were the worst years. Sixty-seven percent of reactions required medical treatment (Plesner and Ronne, 1997). Both of these surveillance studies were very thorough in considering other factors. A general population control would have been useful, but the generally rare incidence of urticaria supported the association with vaccination. In addition, allergic reactions were associated with vaccination in three separate countries. A group in the UK reported no genuine systemic adverse reactions after interviewing respondents to a retrospective questionnaire. The researchers concluded that, "JE vaccines are not associated with a significant risk of systemic allergic reactions" (Nazareth et al., 1994). However, both the interview and questionnaire depended on recall data. Only 21% of questionnaire recipients responded. The study might have missed adverse events through their methodology.

Two other reports detail various neurological disorders in the month following JE vaccination. One Japanese research group reported seven cases of children with acute disseminated encephalomyelitis (ADEM). CT or MRI confirmed the cases. Five of the cases occurred within two weeks of vaccination (Ohtaki et al., 1995). Researchers suspect the association because inactivated JE vaccine is prepared from infected mouse brain. An immune-mediated process might cause post-vaccination ADEM (Ohtaki et al., 1995). Three neurological complications were also reported in Denmark at various times in their vaccination program. The MRI findings supported cases of ADEM. No patients had autoimmune disease or syphilis (Plesner et al., 1996). Other causes are possible, but the timing points to vaccination.

To make a stronger case for association, extremely large studies could compare all adverse effects and neurological complications after vaccination to normal occurrence of adverse events and neurological conditions in the general population. But since these events are so rare, the sample size would have to be huge. Close surveillance is the most feasible method.

Live attenuated vaccine

Although minor symptoms have been reported in association with administration of the live attenuated vaccine, no serious complications have been reported in the administration of over one hundred million doses (Tsai, 1994). A study of 26,239 subjects in Chengdu, China determined the short-term safety of the vaccine. During normal vaccinations in 1995, 13,275 children received the vaccine and 12,964 children did not receive the vaccine. The patients that did not get vaccinated received the vaccine after a one-month observation period. Events as rare as 3 in 10,000 could be detected. No difference was found in incidence of adverse events between the two groups. There were no cases of encephalitis, meningitis, or anaphylaxis. 266 vaccinated subjects were observed on days 1,2,3, and 7 after vaccination. Fever occurred in 13 subjects. No other severe adverse events were observed (Liu et al., 1997). The enormous sample size and existence of a control strengthened this study. But the study period is very short. There is no evidence of long-term safety. In addition, the study is limited to children in an endemic area. Many reported severe adverse effects with inactivated JE vaccination occurred in adults in non-endemic areas.


The study of over 65,000 children in Thailand was one of the most thorough studies on the protective efficacy of the inactivated JE vaccine. The placebo control was key in the convincing power of study. Unfortunately, giving placebo to over 21,000 children with the availability of a widely used vaccine deprived the children of the standard of care. Japan had used the vaccine for over twenty years. It had been licensed in Denmark since 1983. With vaccination, eleven children could have been protected from life-endangering encephalitis. One of the children died and most of the others had severe neurological damage upon examination two or three years later (Hoke et al., 1988). The researchers had a responsibility to vaccinate those children.


The live attenuated vaccine has comparable immunogenicity and protective efficacy to the inactivated vaccine. Its shorter dosage schedule and low production cost ($0.02/dose-$0.03/dose) (Hennessy et al., 1996) make it a desirable option for endemic countries in southeast Asia . The price of the inactivated vaccine is almost $5/dose in Asia compared to $0.75/dose for the live attenuated vaccine (Liu et al., 1997). The next step is to license the live attenuated vaccine in countries outside of China. The vaccine could first be implemented in those countries with endemic JE who could otherwise not afford an extensive JE vaccination program. Long-term studies on efficacy, immunogenicity, and severe adverse effects in non-endemic populations would make it a more convincing option for travelers. In 1998, three doses of JE vaccine cost $192 in the US (web page: ESR, 1998). A cheaper vaccine would be attractive to frequent travelers. Further studies on long-term immunogenic protection would help determine ideal booster schedules. Although careful surveillance can be difficult, it must be maintained to determine the continued safety of each of these vaccines.


Table 1:Comparison between inactivated JE vaccine and live attenuated JE vaccine


Inactivated (Biken)

Live attenuated


Nakayama, Beijing-1



mouse brain

primary hamster kidney cells


1954 – Japan; 1993 – US

1988 – China

Geographic use


traveler vaccination

southeast Asia – childhood







0.5 mL-children, 1.0 mL-adults

Endemic areas

- 2 doses given one to four

weeks apart

- boosters at one year and

every three years as needed

Non-endemic areas

- 3 doses given on days 0, 7, 30

- boosters at one year and

every three years as needed

Endemic areas (China only)

Years 1, 2, 6

- 2 doses given a year apart

- booster given at 6 years


91% efficacy – 2 dose

(95% CI – 70 to 97 percent)

80% efficacy – 1 dose

(95% CI – 44 to 93 percent)

97.5% efficacy – 2 dose

(95% CI – 86 to 99.6 percent)


Antibody levels > or = 1:10

Antibody levels > or = 1:10



77%-100% - 2 dose

85%-100% - 3 dose

76%-100% - 1 dose

94%-100% - 2 dose


Rare cases of urticaria, angioedema, dyspnea, acute disseminated encaphalo-myelitis

No severe adverse effects reported

Possible side effects

Mild rash, fever, vomiting, irritability, diarrhea

Mild rash, fever, vomiting, irritability, diarrhea

Price US$/dose

(in Asia)



Price (in US)

$192 for 3 doses




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