Duration of Neutralizing Antibody Titer after Japanese Encephalitis Vaccination

Changes in age-specific seroprevalence of Japanese encephalitis virus and impact of Japanese encephalitis vaccine in Korea

The Japanese encephalitis (JE) virus is the leading cause of vaccine-preventable encephalitis in Asia. Since the introduction of a universal JE vaccination program and urbanization of Korea, the incidence of JE has dramatically decreased in Korea. However, recent JE cases have occurred, predominantly among unvaccinated adults and with a shift in age distribution. Here we aimed to review the changes in age-specific JE seroprevalence over time and discuss the implications of JE vaccination programs in Korea. Following the last epidemic in 1982-1983, mandatory vaccination for all children aged 3-15 years was conducted annually until 1994. However, JE has reemerged, predominantly affecting unvaccinated adults aged 40 years or older and demonstrating a shift in age distribution toward older populations. The age-specific seroprevalence of the JE virus in Korea has changed noticeably over time. Seropositivity in children and adolescents increased from 10%-59% in the 1970s to 90%-92% in the 1980s after the implementation of the JE vaccination program and increased further to 98% in 2012. No age-specific difference in the seroprevalence of JE was found, and appropriate levels of immunity to JE were maintained for all age groups. Continuous surveillance of the seroprevalence of JE is essential to establish a proper immunization policy in Korea.

Japanese encephalitis vaccines: Immunogenicity, protective efficacy, effectiveness, and impact on the burden of disease

Japanese encephalitis (JE) is a serious public health concern in most of Asia. The disease is caused by JE virus (JEV), a flavivirus transmitted by Culex mosquitoes. Several vaccines have been developed to control JE in endemic areas as well as to protect travelers and military personnel who visit or are commissioned from non-endemic to endemic areas. The vaccines include inactivated vaccines produced in mouse brain or cell cultures, live attenuated vaccines, and a chimeric vaccine based on the live attenuated yellow fever virus 17D vaccine strain. All the marketed vaccines belong to the JEV genotype III, but have been shown to be efficacious against other genotypes and strains, with varying degrees of cross-neutralization, albeit at levels deemed to be protective. The protective responses have been shown to last three or more years, depending on the type of vaccine and the number of doses. This review presents a brief account of the different JE vaccines, their immunogenicity and protective ability, and the impact of JE vaccines in reducing the burden of disease in endemic countries.

Modeling the long-term persistence of hepatitis A antibody after a two-dose vaccination schedule in Argentinean children

BACKGROUND

Long-term seroprotection data are essential for decision-making on the need and timing of vaccine boosters. Based on data from longitudinal serological studies, modeling can provide estimates on long-term antibody persistence and inform such decision-making.

METHODS

We examined long-term anti-hepatitis A virus (anti-HAV) antibody persistence in Argentinean children ≤15 years after the initial study where they completed a 2-dose course of inactivated hepatitis A vaccine (Avaxim 80U Pediatric, Sanofi Pasteur, Lyon, France). Blood serum samples were taken at baseline, 2 weeks (post first dose), 6 months (pre-booster), 6.5 months (post-booster), 10 years and 14-15 years after first vaccine dose. We fitted 8 statistical model types, predominantly mixed effects models, to anti-HAV persistence data, to identify the most appropriate and best fitting models for our data set and to predict individuals' anti-HAV levels and seroprotection rates up to 30 years post vaccination.

RESULTS

Fifty-four children (mean age at enrollment 30.4 months) were enrolled up to 15 years post first vaccine dose. There were 3 distinct periods of antibody concentration: rapid rise up to peak concentration post-booster, rapid decay from post-booster to 10 years, followed by slower decay. A 3-segmented linear mixed effects model was the most appropriate for the data set. Extrapolating based on the available 14-15-year follow-up, the analysis predicted that 88% of individuals anti-HAV seronegative prior to vaccination would remain seroprotected at 30 years post vaccination and lifelong seroprotection for vaccinees seropositive prior to vaccination.

CONCLUSIONS

Currently available data demonstrate that Avaxim 80U Pediatric confers to most vaccinees a high level of seroprotection against hepatitis A infection for at least 20-30 years.

Comparison of four serological tests for detecting antibodies to Japanese encephalitis virus after vaccination in children

Objectives

Several different methods are currently used to detect antibodies to Japanese encephalitis virus (JEV) in serum samples or cerebrospinal fluid. These methods include the plaque reduction neutralization test (PRNT), the hemagglutination inhibition (HI) test, indirect immunofluorescence assay (IFA), and enzyme-linked immunosorbent assay (ELISA). The purpose of this study was to compare the performance of each method in detecting vaccine-induced antibodies to JEV.

Methods

The study included 29 children who had completed a primary immunization schedule with an inactivated vaccine against JEV derived from mouse brain (n = 15) or a live attenuated SA14-14-2 vaccine (n = 14). Serum samples were collected between 3 months and 47 months after the last immunization. The serum samples were tested by performing the PRNT, HI test, in-house IFA, and commercial ELISA. The antibody detection rates were compared between tests.

Results

All 29 serum samples were positive with the PRNT, showing antibody titers from 1:20 to 1:2560. The HI test showed positive rates of 86.7% (13/15) and 71.4% (10/14) in the inactivated and live attenuated vaccine groups, respectively. The results of the IFA for immunoglobulin (Ig)G were positive in 53.3% (8/15) of children in the inactivated vaccine group and 35.7% (5/14) in the live attenuated vaccine group. Neither the IFA nor ELISA detected JEV IgM antibodies in any of the 29 children.

Conclusion

These results show that detection rates of vaccine-induced antibodies to JEV have a wide range (0–100%) depending on the testing method as well as the time since immunization and individual differences between children. These findings are helpful in interpreting serological test results for the diagnosis of Japanese encephalitis in situations where vaccines are widely administered.

Immunogenicity of single-dose Vero cell-derived Japanese encephalitis vaccine in Japanese adults

In Japan, intensive immunization against Japanese encephalitis (JE) was performed from 1967 to 1976, and regular JE immunization was performed thereafter. However, for Japanese adults facing JE risk, dates of vaccination with new inactivated Vero cell-derived JE vaccine are unavailable. This study investigated how a single dose of Vero cell-derived JE vaccine affects Japanese adults. Neutralizing antibodies were measured pre- and post-JE vaccination in 79 participants (age 40.7 ± 9.4 years), enrolled between October 2009 and March 2011, whose JE-vaccination data were gathered from vaccination records and history taking. Before vaccination, the participants' seroprotection rate (SPR) was 51.9%, whereas SPR after vaccination was 93.7%. The seroconversion rate (SCR), which measures seronegative cases that turn seropositive after vaccination, was 86.8%. The geometric mean titer (GMT) was 14.7 before vaccination and 70.1 after vaccination. Age was a significant difference between seroprotected (42.8 years) and non-seroprotected (38.7 years) groups before vaccination. Then the difference of age, SCR, pre-vaccination GMT, post-vaccination GMT and sex ratio were also significant in participants aged 25-39 years and ≥40 years, who represent generations born when Japan's JE-vaccination policy changed. SCR was 100% in participants aged 25-39 years with a vaccination recorded 55.6% in participants aged 25-39 without a vaccination record, and 96.0% in participants aged ≥40 years. Thus, more participants aged 25-39 years were seroprotected before vaccination, but SCR was higher in those aged ≥40 years. Most Japanese adults can be protected after one-dose vaccination, but this may be insufficient for people aged 25-39 years without recorded JE vaccination.

A 3-year follow-up of antibody response in HIV-infected children with immune recovery vaccinated with inactivated Japanese encephalitis vaccine

Among HIV-infected children who had immune recovery after received antiretroviral therapy (ART), good responses to revaccination with childhood vaccines have been observed. However, the rate of long-term persistence of antibody response remains unknown. The objective of this study is to determine whether HIV-infected children still have protective antibody against Japanese encephalitis virus (JE) 3 years after receiving revaccination with two doses of inactivated JE vaccine. Plasma JE neutralizing antibody titer was determined by a plaque reduction neutralization assay. An antibody titer of more than 1:10 was defined as being protective. Fifty HIV-infected children with a mean age of 10.3 years (SD 2.2) and mean CD4 percentage of 25 (SD 5) were revaccinated with two doses of inactivated JE vaccine. Forty-three children had been followed-up for 3 years. The JE neutralizing antibody at 1 month and 3 years after revaccination were detected among 38 (88%) and 35 (81%) children, respectively. The geometric means titer significantly dropped from of 306 (min 13-max 163,617) to 106 (min 11-max 4645). This data show that the majority of HIV-infected children had persistent antibody 3 years after revaccination. JE revaccination in HIV-infected children with immune recovery after ART should be carried out in endemic areas.

IC51 Japanese encephalitis vaccine

Japanese encephalitis is the leading cause of viral encephalitis in Asia. Every year 30,000 - 50,000 cases and 10,000 deaths from Japanese encephalitis are reported, and underreporting has been suggested. No effective antiviral therapy exists to treat this mosquito-borne flavivirus infection. For active immunization vaccines are available. The manufacturing of the only vaccine that was internationally licensed, JE-VAX, was ceased in 2005. Therefore a shortage of Japanese encephalitis vaccines might occur before new generation vaccines based on cell culture technology will be available. A promising new vaccine candidate is the inactivated whole-virus vaccine IXIARO (Strain SA(14)-14-2), developed by Intercell AG. Which was licensed in Europe, the USA and Australia in spring 2009. Recently, successful Phase III immunogenicity and tolerability studies were published, indicating that this vaccine will be an acceptable approach to active immunization against Japanese encephalitis. Cell-culture-based vaccines will not only be used in the population living in endemic areas where the risk of infection is high, but also by travelers and military personnel.

Current use and development of vaccines for Japanese encephalitis

BACKGROUND

Japanese encephalitis (JE) is a significant cause of human morbidity and mortality throughout Asia. Vaccines for JE have been available for many years and their use has been effective in reducing the incidence of JE disease in several countries but, as disease incidence has decreased, concerns regarding adverse events following immunisation have increased.

OBJECTIVE

To review existing JE vaccines and new candidates in advanced preclinical or clinical evaluation.

METHODS

The review primarily covers published and some unpublished literature from the past decade describing current use of approved JE vaccines in various parts of the world, and advanced development and clinical testing of alternative vaccine candidates.

RESULTS/CONCLUSION

There is a clear need for additional licensing of existing or new JE vaccines. Several promising candidates are currently in use or completing clinical trials.