Product review on the JE vaccine IXIARO

Abstract

Japanese encephalitis virus, as the most important vaccine-preventable cause of viral encephalitis in Asia, is estimated to cause over 68,000 clinical cases yearly. In endemic areas, most Japanese encephalitis infections occur in children younger than 10 y and clinical manifestation of this disease is critical, because there is no effective treatment available. As JEV infections are regarded as one of the most serious viral causes of encephalitis and mass immunization programmes are generally recommended for residents in endemic areas, a safe and effective JEV vaccine was needed to protect them as well as others at risk. Due to the safety concerns with the mouse brain derived vaccine, second generation vaccines against JE produced in cell culture like Vero cells were developed. IXIARO® is a purified, inactivated aluminum-adjuvanted JE vaccine, based on the SA14–14–2 virus strain, and is available in North America, Europe, Canada, Switzerland, Singapore, Hong Kong and Israel as well as in Australia & New Zealand (as JESPECT®).The safety, tolerability and immunogenicity profile of IXIARO® is well established through a number of clinical studies comparing IXIARO® with placebo as well as mouse brain derived vaccine. Recent data show that the global incidence of JE remains substantial, especially young children in endemic areas are most susceptible. As vaccination is the most feasible, reliable and cost effective tool for JE control, IXIARO® with confirmed excellent safety profile is highly recommendable, in particular for vaccination of children at risk. The European Commission as well as the FDA approved the extension of indication of IXIARO® to the pediatric segment (2 months of age and older) based on these data.

Keywords:

• Japanese encephalitis virus
• inactivated vaccine

Introduction

Japanese encephalitis virus (JEV) is a single-stranded RNA virus that belongs to the genus Flavivirus and is closely related to West Nile, St. Louis encephalitis, yellow fever, and dengue viruses.^1,2 Five genotypes of JEV have been identified,³ all currently available vaccines are derived from strains of genotype GIII, formerly the predominant genotype in large areas of Asia.⁴ As the natural hosts of the flavivirus are domestic pigs and water birds, an eradication of the mosquito-borne virus is very unlikely due to this animal reservoir, so active vaccination remains the most important control measure worldwide.⁵

Japanese encephalitis virus, as the most important vaccine-preventable cause of viral encephalitis in Asia, is estimated to cause over 68,000 clinical cases yearly.^6-8 Highly endemic in Southeast Asia, India, China, Japan and Korea, the virus has expanded its distribution including parts of Oceania,⁴ with approximately 3 billion people at risk for infection. Sporadic epidemics in less endemic areas also account for tens of thousands of cases annually, and new foci of epidemics have been noted in formerly non-endemic areas, such as Sri Lanka or Nepal.⁹ The reasons for this increased geographic distribution are uncertain but might include population shifts or changes in climate, ecology, agricultural practices, animal husbandry, or migratory bird patterns.^10,11,12 These factors could contribute to further spread, including potentially beyond Asia and the western Pacific.¹³

JEV is transmitted to humans through the bites of infected mosquitoes. Humans usually do not develop a level or duration of viremia sufficient to infect mosquitoes.^5,14 Therefore, humans are dead-end hosts, and human JE cases imported into nonendemic areas represent a minimal risk for subsequent transmission of the virus.¹³

In endemic areas, most Japanese encephalitis infections occur in children younger than 10 years,^6,7 but non-immune adults who live in or visit endemic areas can also be affected.

The great majority of infections are asymptomatic, and acute encephalitis, the most commonly identified clinical syndrome with JEV infection, occurs only in 1 of every 50 to 1000 infected individuals,⁵ but clinical manifestation of this disease is critical, because there is no effective treatment available. Symptoms of JE disease typically start with a sudden onset of fever, chills and aches, including headaches and are complicated by neurological signs and symptoms, culminating in progressive coma, due to neuroinvasion of the JEV.¹⁵

Mental status changes, focal neurologic deficits, generalized weakness, and movement disorders might occur over the next few days and seizures are common, especially among children.¹⁶ The classical description of JE includes a Parkinsonian syndrome with mask-like facies, tremor, cogwheel rigidity, and choreoathetoid movements.¹⁷ JE may present as mild or moderate disease leading to an uneventful recovery or may rapidly progress to severe encephalitis. Status epilepticus, brain hypoxia, increased intracranial pressure, brainstem herniation, and aspiration pneumonia are the most common complications associated with poor outcome and death.^16,17 Up to 30% of these JE-infected individuals die and 30–50% of survivors have significant neurologic or psychiatric sequelae requiring rehabilitation and continued care.^18,19 So, within one month, JE killed more than 1200 children during an epidemic outbreak in Uttar Pradesh, India and Nepal.²⁰

The immune response to JEV infection has not been fully characterized, and both humoral and cellular responses may play a role.^21,22 However, it is widely accepted that virus neutralizing antibodies provide the best evidence that protective immunity against JEV has been established and WHO recognizes a neutralizing antibody titer of ≥1 :10 determined in a Plaque Reduction Neutralization Test as protective.^23,24

Background

Vaccines against the JEV infection were developed since the 1930s in Russia and Japan. Several first-generation, inactivated vaccines, with mouse brain as a substrate for growth of the virus, have been produced by Japanese, Korean, Vietnamese, and other national manufacturers for decades. Later on, Chinese manufacturers started producing both inactivated and live-attenuated virus vaccines using primary hamster kidney cells for virus propagation.^25-28 The use of these vaccines has not remained limited to domestic supply, in addition, there has been utilization of the live attenuated SA14–14–2 JE vaccine, which was recently prequalified by the World Health Organization, in national immunization programs in Asia.²⁹ However, potential safety hazards with live vaccines remain a constant source of controversy in the scientific community and among public-health officials,^25,30,31 with the potential threat of mutagenic reversion into pathogenic strains as the greatest fear among critics.

JE-VAX®, licensed since 1992 and the only licensed vaccine in the USA, Canada and Australia in the 1990ies and early 2000s, was a formalin-inactivated, mouse-brain-derived vaccine, manufactured by the Foundation for Microbial Diseases of Osaka University in Japan with proven protective efficacy.³² Serious side-effects, such as anaphylaxis occurring typically delayed 1–10 d (up to 17 days) after vaccination, had been noted with an incidence of 15–62 per 10 000 people in the USA. Although the exact cause of these reactions was unknown, most experts regarded the porcine gelatin stabilisers included in the formulation of the licensed vaccine to be responsible for these severe side-effects. Furthermore, neural tissue content of the vaccine had raised concerns about safety and the possibility of vaccine-related neurological side-effects.³³ Cases of disseminated encephalitis in Japanese children had been suggested to be related to vaccination with mouse-brain-derived JEV vaccine, which led to a suspension of the recommendation to use this vaccine type in the Japanese infant immunisation program.³⁴ These safety concerns led to the suspension of routine vaccination with mouse-brain-derived inactivated JEV vaccines in Japan in May, 2005³⁵ and in 2006 production of JE-VAX® was discontinued.

As JEV infections are regarded as one of the most serious viral causes of encephalitis and mass immunisation programmes are generally recommended for residents in endemic areas, a safe and effective JEV vaccine was needed to protect them as well as others at risk, like travelers and military personnel.³⁶

Due to the safety concerns with the mouse brain derived vaccine, second generation vaccines against JE produced in cell culture like Vero cells were developed independently in Europe (by Intercell AG, now Valneva SE) and Japan (by Biken and Kaketsuken).^37,38 Although all these vaccines are produced in Vero cells, there are significant differences in the vaccines, regarding strain of JE, manufacturing steps and formulation of the final product.

Valneva's JE vaccine was named JE-PIV in the phase 1 and 2 studies while IC51 was the development code used for the vaccine in all further trials; IXIARO® is the trade name of the licensed product.

Production

The study vaccine used in the phase 1 and 2 studies was a JE purified, inactivated virus (JE-PIV) vaccine, lot 0737, developed and manufactured by the Pilot Bioproduction Facility, Walter Reed Army Institute of Research (WRAIR), Forest Glen, MD 20910, USA. The attenuated SA14–14–2 vaccine strain, adapted to primary canine kidney cells,³⁹ was further passaged in Vero cells. Vero cells used for production were a derivative of a certified cell line that has been used to produce more than 1 billion doses of licensed polio and rabies vaccines.⁴⁰ After adaptation to Vero cells, a master seed (Vero- 4) and production seed (Vero-5) were prepared and banked at −80°C. A vaccine lot was prepared at Vero passage 6 by inoculation of Vero cells grown in 850 cm2 roller bottles.

After inoculation, virus was harvested on Days 3, 5, 7 and 9. A total of 10 L of harvested virus was pooled and clarified by centrifugation followed by filtration and ultrafiltration (100,000 MWCO). To remove Vero cell DNA, the concentrated virus was treated with protamine sulfate, clarified and purified by zonal centrifugation in sucrose gradients. Gradient fractions were assayed for antigen activity associated with viral particles. Antigen positive fractions were pooled, diluted and formalin added at a concentration of 0.05% (v/v). After 10 d inactivation at 22°C, formalin was neutralized with sodium bisulfite and the bulk vaccine stored at 4°C.

Intercell sublicensed the rights to manufacture the vaccine from VaccGen International LLC, USA and transferred the process from WRAIR to Intercell's manufacturing facility in Livingston, Scotland. Phase 3 clinical materials were prepared in this facility. Several changes were performed between the WRAIR production process and the Phase 3 and Commercial Process.

The JEV test vaccine IC51 (Intercell Biomedical, Livingston, UK) used in all further trials was a purified inactivated vaccine, containing the JEV strain SA14–14–2. This attenuated strain was propagated in Vero cells. The vaccine was prepared by a purification and inactivation process consistent with current good manufacturing practices. The finished product does not contain thimerosal or gelatins.³⁶ The Final medicinal product bulk from the commercial scale production process is produced at Intercell Biomedical Limited, Livingston UK but filled into single dose syringes and packaged at Vetter Pharma-Fertigung GmbH & Co. KG, Ravensburg, Germany.

IXIARO® is presented as a ready-to-use suspension for injection in a single-dose pre-filled syringe. Each dose of 0.5 ml contains JEV target total protein concentration of 6 μg that corresponds to a potency of ≤ 460 ng ED₅₀, adsorbed to 0.1% alumimium hydroxide. The vaccine is stored at 2˚C–8˚C with a proposed shelflife period of 12 months.

Assays

Pre-clinical testing consisted of tests for adventitious microbial agents, mycoplasma, cellular protein and DNA contaminants, endotoxin, reverse transcriptase (PERT assay), mouse immunogenicity and efficacy, and viral-specific protein and antigen. All test results were satisfactory and met specifications. Vero-specific cellular DNA was measured at 2 pcg/mL. For the final container vaccine lot no. 0737, the bulk vaccine was adsorbed to alum (Rehydragel), filled in 0.7mL vials and stored at 2–8 °C. Final container vaccine was tested for sterility, pH, aluminum content, residual bisulfite, pyrogen and identity. All test results were satisfactory and met specifications. For dosing, each dose was formulated to contain either 6.0 or 12.0 mg per dose.⁴¹

Preclinical Studies

The preclinical toxicity data for IXIARO® includes one formal Good Laboratory Practice (GLP) developmental and reproductive toxicity study. Pharmacological data submitted in support of IXIARO® licensure were limited to primary pharmacodynamic studies. This reduced testing approach was justified and agreed by the European Medicines Agency (EMA) (EMEA/CHMP/SAWP/146087/2005).

Pharmacodynamic studies peformed with IXIARO® included immunogenicity studies in mice, rats and rabbits. With the exception of a WRAIR's mice protection study using early batches of decreased potency, all pharmacological studies used vaccine lots manufactured using Intercell's process, as were used in the phase 3 trials.

Vaccinated animals developed high-titered JE virus neutralizing (i.e. protective) antibodies in a dose dependent fashion after 2 injections with a PRNT₅₀ range from 1280 to 7781 in the highest dose (800ng). The lowest dose of the JE-PIV tested that elicited neutralizing antibody in 100% of vaccinated mice with Geometric Mean Titers of 113, was 32 ng. The vaccine protected mice against morbidity and mortality after challenge with live, virulent, JE virus. Compared with the existing licensed mouse brain-derived vaccine, the Vero cell-derived JE-PIV was more immunogenic and as effective as JE-VAX® in preventing encephalitis in mice.⁴²

Results from the other studies showed that vaccination with IXIARO® induced high levels of JEV-specific neutralizing antibodies in all animals, with peak levels measured at day 35 and without significant difference between rats and rabbits.

As efficacy trials to measure vaccine efficacy are technically difficult, quantification of neutralizing antibody is a proposed surrogate measure of vaccine efficacy. It is generally thought that the presence of JEV neutralizing antibody titers of at least 1:10 correlates with immunity to JE.⁴³

A mouse JEV challenge model was used to assess the correlation between IXIARO®-induced JEV neutralizing antibody titers and protection.⁴⁴ In this study, animals were actively immunized by vaccination or passively immunized with sera from volunteers, who were vaccinated in a blinded Phase 3 clinical trial with JE-VAX® or IXIARO®, then challenged with viruses representing dominant homologous and heterologous genotypes to establish a protective threshold antibody titer.

Sera from IXIARO® vaccinated subjects were pooled into 4 batches based on neutralizing antibody titers measured by plaque reduction neutralization test (PRNT50 titer): high (∼200), medium (∼40–50), low (∼20) and negative (<10 ). Pooled sera from JE-VAX® vaccinated subjects (PRNT50 titer ∼55) and pooled JEV antibody negative pre-vaccination sera were used as controls. Groups of 10 6- to 7-week-old female ICR mice were injected intraperitoneally with 0.5 ml of each serum pool diluted 1:2 or 1:10, challenged approximately 18 h later with a lethal dose of either JEV strain SA14 (genotype III) or strain KE-093 (genotype I) and observed for 21 d. All mice in the non-immune serum groups developed clinical signs consistent with JEV infection or died, whereas passive immunization with pooled high titer sera from both IXIARO® and JE-VAX® sera protected 90–100% of the animals. Statistical tests showed similar protection against both JEV strains SA14 and KE-093 and protection correlated with the anti-JEV antibody titer of IXIARO® sera as measured by PRNT50. Ex vivo neutralizing antibody titers showed that almost all mice with a titer of 10 or greater were fully protected. In a separate study, analysis of geometric mean titers (GMTs) of the groups of mice vaccinated with different doses of IXIARO® and challenged with JEV SA14 provided additional evidence that titers ≥ 10 were protective. Additionally, antibodies generated against IXIARO® protected not only against JEV genotype III but as well against genotype I virus.⁴⁴ A summary of clinical trials is provided in Table 1.

Clinical Studies

Phase I

In a Phase I trial, the JE-PIV was shown to be safe in subjects and stimulated immune responses in approximately 50% of vaccinees, presumably due to the low (0.5 and 2.0 mg) doses given in the study. (unpublished data)

Phase II

In a phase II randomized, open label, unblinded, single center trial a total of 94 eligible subjects were allocated into one of 4 groups and vaccinated sequentially.⁴¹ The purpose of this study was to show that JE-PIV is sufficiently immunogenic and well tolerated in the doses and schedules tested. Group 1 (24 subjects) received JE-PIV (6 mg/dose) on Days 0 and 28 (12 mg total dose); Group 2 (24 subjects) received JE-PIV (6 mg/dose) on Days 0, 14 and 28 (18 mg total dose); Group 3 (25 subjects) received JE-PIV (12 mg/dose) on Days 0 and 28 (24 mg total dose); and Group 4 (21 subjects) received mouse-brain derived JE-VAX® subcutaneously (SQ) according to the recommended schedule on Days 0, 7 and 28. The 6 and 12 mg doses of JE-PIV were selected because they best approximated the amount of protein in JE-VAX® (approximately 6 mg per dose). Each JE-PIV dose was given at least 2 weeks apart, because a 1-week interval between doses was deemed inappropriate for a vaccine containing alum. In general, all dose regimens with JE-PIV resulted in higher seroconversion rates compared to JEVAX ®. High rates of seroconversion at Day 56, the primary immunogenicity endpoint, were observed for all 3 JE-PIV treatment groups. In summary, this Phase 2 trial showed that JE-PIV in 3 different dose regimens was tolerable and induced good immune responses to JEV. This immune response appeared to be durable, with robust sustained GMTs over at least 2 y The results of this study suggested that the JE-PIV vaccine could be considered a good candidate for a replacement of mouse brain derived JE vaccine.

In endemic regions the most frequently affected age group is children, depending on the country children under 9 and 14 y⁹ Hence immunization should start early in childhood. JE vaccines like JE-VAX® or JenceVacTM were approved for pediatric use and administered to small children starting at the age of 1 y In a Phase II trial in 60 healthy Indian children between 1 and 3 y of age immunogenicity and safety of 3 and 6μg of IC51 compared to JenceVacTM were investigated. JE specific neutralizing antibodies were measured at baseline and 28 d after the first and second vaccination. On Day 56 seroconversion rates (SCR) of the 3 and 6μg IC51 and the JenceVacTM group were 95.7%, 95.2% and 90.9%, respectively, and geometric mean titers (GMT) were 201, 218 and 230, respectively, without statistically significant differences between the 3 groups. Local and systemic tolerability were captured in a diary 7 d post-vaccination. No apparent difference was seen in the safety profile between the vaccines. These first immunogenicity and safety data in children were promising and supported the use of a 3μg dose in children younger than 3 y for further development of IC51 in the pediatric population.⁴⁵

Phase III

In a non-inferiority, phase III, multicentre, multinational, observer-blinded trial 867 adults were included.³⁶ Study sites were located in the USA, Germany, and Austria. Volunteers received either the JEV test vaccine IC51 intramuscularly on a 2-dose schedule (on days 0 and 28; n = 430 ) or the licensed vaccine JE-VAX® subcutaneously according to its recommended 3-dose schedule (on days 0, 7, and 28; n = 437 ). The primary endpoint was immunogenicity, with respect to neutralising JEV-specific antibodies assessed by a plaque-reduction neutralisation test, which was assessable in 725 patients in the per-protocol population. The safety profile of the test vaccine was good, and its local tolerability profile was more favorable than that of the licensed vaccine. Frequency of adverse events was similar between treatment groups, and vaccine-related adverse events were generally mild. The seroconversion rate of the test vaccine was 98% compared with 95% for the licensed vaccine on day 56 (95% CI for the difference –1.33 to 3.43). The Geometric mean titer for recipients of the test vaccine was 244 (range 5–19 783), compared with 102 (5–1864) for the licensed vaccine (ratio 2.3 [95% CI 1.97–2.75]).

The aim of a large randomized, double-blind, placebo-controlled multicenter phase III trial was to evaluate the safety and tolerability of IC51. Healthy subjects were randomized to receive 2 doses of IC51 (n = 2012) or placebo (n = 663) at a 4-week interval. Adverse events following immunization (AEFI) were documented over a period of 2 months. The rate of serious AEFI was similar in the IC51 group (0.5%) and the placebo group (0.9%). The rate of medically attended AEFI and all AEFI was also similar in the IC51 group and the placebo group. The same applied for all adverse events, including local and systemic tolerability. Importantly, there were no signs of acute allergic reactions. The Intercell JE vaccine IC51 had a safety profile similar to that of placebo. These data, together with the immunogenicity data from the preceding phase 3 trial (see 3.1), formed the basis of application for licensure of this vaccine.⁴⁶

A multicenter follow-up phase III study examined the long-term immunogenicity of IC51.⁴⁷ Adult subjects from 2 studies ^36,46 were followed-up for comparative immunogenicity (JE-VAX®) at 6 months and long-term immunogenicity of IC51 alone at 12 months. At 6 months, immunogenicity was higher with IC51 (seroconversion rate [SCR] 95%; geometric mean titer [GMT] 84) than with JE-VAX® (SCR 74%; GMT 34). At 12 months, the SCR was 83% and the GMT remained above the protective titer of 1:10. Overall, the safety profile of IC51 was excellent, and comparable to placebo, which is essential for a vaccine for healthy recipients.⁴⁷ The AE and SAE frequencies were similar at 6 months for IC51, JEVAX ®, and placebo as were the frequencies for the most common AEs (<5 %) of nasopharyngitis, influenza-like illness and headache. The safety profile for IC51 was consistent at 12 months. Further follow-up for a total of 5 y showed stabilization of the immune response at the level seen after 12 months in this cohort of subjects.⁴⁸

The standard administration schedule of IC51 is 2 doses of 6μg with a 28-day interval. A randomized, observer-blind, controlled Phase III study investigated the immunogenicity of a single-immunization, high-dose regimen (1×12 μg) compared to the 2-injection, standard regimen to determine the immune response that one, high-dose injection can confer. The single, high-dose regimen resulted in about 60% seroconversion rate at 10 d after administration, but it did not reach the almost 100% SCR already achieved by the 2-dose standard administration 7 d after the second immunization.⁴⁹

The following multi-center, open-label, phase III follow-up trial was designed to assess the long-term immunogenicity of primary IC51 vaccination and the immune response to an IC51 booster dose in subjects without protective neutralizing antibody titers. Subjects had received primary IC51 vaccination in a preceding 2 month, randomized, controlled, observer-blinded, phase 3 trial (see 3.4.), comprising either a complete 2-dose series (2 doses of 6mcg, Days 0 and 28) or one of 2 single-dose regimens (6mcg or 12mcg on Day 0 and sham injections on Day 0 [6mcg group only] and Day 28). In this follow-up trial, immunogenicity was assessed at 6, 12 and 24 months after first vaccination. Subjects with a negative plaque reduction neutralization test (PRNT) result at 6 and/or 12 months after first IC51 vaccination received a booster IC51 6mcg dose at 11 and/or 23 months after first vaccination. After primary immunization with the Day 0/28 dose schedule, seroprotection rates were 83%, 58% and 48% at Month 6, Month 12 and Month 24, respectively. Hence, a booster may be needed from 12 months after the primary immunization onwards. A booster dose at Month 11 and/or Month 23 in subjects with neutralizing antibody titers below the limit of detection ([PRNT50] < 1:10) led to 100% seroconversion. After a single-dose immunization (incomplete primary immunization), only 9% of subjects were seroprotected at Month 6; however, a booster dose at Month 11 led to seroconversion in 99% of subjects. This demonstrated that IC51 is able to induce immunological memory and that booster doses trigger an anamnestic immune response. Hence, subjects with incomplete primary immunization can complete their schedule within at least 11 months. The standard 2-dose regimen yielded high seroprotection rates up to 6 months after primary immunization in this trial. One year later a booster dose might be necessary if exposure to JE remains a risk. To determine the optimal timing of a booster interval for these individuals, further trials were conducted.⁵⁰

As data on the immunogenicity of booster vaccinations for licensed JE-vaccines are rare, it was of interest to examine the effects of a booster vaccination on the immunogenicity and safety of IXIARO. In this phase III trial, 198 subjects, who had received primary immunization in a preceding randomized trial, were boosted with IXIARO 15 months after the primary immunization. Neutralizing antibody titers were assessed by plaque-reduction neutralisation test, PRNT. Prior to the booster dose, 69.2% (137/198) of subjects had PRNT50 titers ≥ 1:10. One month after the booster, the rate of subjects with PRNT50 ≥ 1:10 (recognized as a protective titer) was 100%. This rate remained high at 98.5% at 6 and 12 months; GMTs were 22.5 before the booster and 900, 487 and 361 at 1, 6 and 12 months after the booster, respectively. A booster dose of IXIARO at 15 months after primary immunization was highly immunogenic with GMTs >fold5- higher than those seen immediately after primary immunization, and remained at high levels for at least 12 months after the booster. Additional data regarding long-term immunity following primary immunization with the 2-dose regimen of IXIARO will be derived from a still ongoing long-term-follow-up phase III study, which recently has completed year 3 after immunization.⁵¹

As several pre-departure immunizations are often indicated in travelers, data were needed about possible interactions between vaccines. A single-blind, randomized, controlled Phase III trial investigated the immunogenicity and safety of IC51 (JE vaccine) and HAVRIX®1440 (hepatitis A vaccine) when administered alone or concomitantly to healthy subjects. The immune response was compared between single and concomitant vaccination in terms of geometric mean titer (GMT) and seroconversion rate on Days 28 and 56. Immunogenicity was comparable for the 2 vaccines whether given together or separately which suggested that travelers could receive such vaccinations concomitantly.⁵²

An overview of the safety and tolerability of IC51, licensed as IXIARO®, for 6 months after the first vaccination in 7 Phase III trials, indicated that IXIARO® had a favorable safety profile, comparable to PBS + Alum control vaccination and appeared to have a better local tolerability profile than JE-VAX®. 3558 subjects with at least one IXIARO® vaccination were included, 435 subjects with a JE-VAX® (manufactured by BIKEN, distributed by Sanofi Pasteur) vaccination, and 657 with phosphate buffered saline solution with 0.1% Al(OH)3 (PBS + Alum) control vaccination. The percentage of subjects reporting solicited local adverse events (AEs) with IXIARO® (54%) was similar to PBS + Alum vaccination (56%) as were solicited systemic adverse events (40% IXIARO®; 40% PBS + Alum vaccination). Vaccination with JE-VAX® resulted in a higher frequency of solicited local adverse events (61%) but in a slightly lower frequency of solicited systemic adverse events (36%). The frequency of subjects with any solicited and unsolicited AE with IXIARO® (64%) was also similar to PBS + Alum vaccination (61%) and JE-VAX® (64%); as for subjects with serious AEs (1% IXIARO®; 2% PBS + Alum vaccination, 1% JE-VAX®). No serious allergic reactions were observed in any group.⁵³

Regulatory status

The clinical development program to support licensure of IXIARO® in adults consisted of 9 studies in which approx. 4,700 subjects were enrolled and vaccinated. IXIARO® is licensed in the European Economic Area (EEA) and USA since March 2009. The product is also approved in Australia & New Zealand (as JESPECT), Canada, Switzerland, Singapore, Hong Kong and Israel. In all these territories it was initially indicated for active immunization against Japanese encephalitis in adults from 18 y (in the US, from 17 years) onwards.

In addition to the adult indication, the vaccine has been approved for use in adolescents, children and infants aged 2 months and older in 2013 in EEA, USA, Singapore and Hong Kong and Israel.

Product availability

IXIARO® is a prescription medicine generally available in travel clinics in countries where the product is authorized. There are no reports from FDA or EMA regarding product shortage.

Public-health

For most travelers to Asia, the risk for JE is very low but varies based on destination, duration, season, and activities. The Advisory Committee on Immunization Practices (ACIP) recommends JE vaccine for travelers who plan to spend a month or longer in JE-endemic areas during the JE virus transmission season. JE vaccine should also be considered for short-term travelers (<1 month) if they plan to travel outside of an urban area and have an itinerary or activities that will increase the risk of JE virus exposure and for laboratory personnel with a potential for exposure to infectious JE virus.⁵⁴ In a recent review of JE cases in travelers, a detailed risk assessment of 37 cases revealed that 35% had spent less than 1 month in JE-endemic areas and thus would not have been recommended vaccination.⁵⁵ Nonetheless IXIARO® provides proven protection with a favorable safety profile and outdated guidelines and recommendations should be revised according to an expert recommendation: travelers to rural areas of Asia should generally be recommended vaccination.⁵⁶

For travelers, a primary immunization regimen of 28 d can take too long to complete before departure. Hence, an accelerated JE dosing regimen (vaccination within one week) was recently evaluated in a phase III randomized, observer-blind study aiming to establish non-inferiority of the immune response to an accelerated Day 0, 7 dosing regimen of IXIARO [administered concomitantly with a purified chick-embryo cell rabies vaccine], as compared to the conventional schedule. The study met its endpoints and the accelerated schedule resulted in a SCR of 99% from 7 d after the second dose on, with higher titers than the standard schedule, and a generally comparable safety profile. This accelerated regimen could be a valid alternative schedule for the prophylaxis of JE.⁵⁷

With respect to possible combinations, Rabies vaccine is considered the most important concomitantly administered vaccine, because the profile of travelers who are recommended JE and Rabies pre-exposure prophylaxis overlap in many aspects (e.g. prolonged rural exposure, hiking, farming, hunting, camping etc.). Interference between IXIARO® and Rabipur® (Rabies vaccine) administered with or without a MenACWY-CRM (Menveo®) vaccine was recently under investigation in a phase IIIb study in healthy adults.⁵⁸

The results of this study demonstrate that IXIARO® can be co-administered with the quadrivalent meningococcal vaccine (Menveo®) and the Rabies vaccine Rabipur® without comprising antibody responses or safety profiles. This can aid in the development of a more inclusive protection plan for travelers, reducing the need for multiple visits without compromising immunogenicity or safety.

For adults, it also has been demonstrated that Hepatitis A vaccine does not interfere with the response to IXIARO® or vice versa.

Pre-existing TBE antibodies have shown to increase JE virus neutralizing antibody titers after the first IXIARO® vaccination. This was investigated in an active-controlled, randomized, multi-center, observer blinded, phase III trial, measuring the neutralising antibody response to IC51 in subjects with (N = 81) and without (N = 339) pre-existing tick-borne encephalitis (TBE) vaccine induced antibodies as determined by TBE enzyme-linked immunosorbent assay IgG (ELISA). Neutralising antibody response was statistically superior in TBE ELISA-positive subjects compared to TBE ELISA-negative subjects after the first (p < 0.0001) but not after the second vaccination with IC51. Thus, pre-existing vaccine-induced TBE immunity enhanced the neutralising JEV-specific antibody response after the first IC51 vaccination.⁵⁹

More than 10 000 cases of JE are still reported each year in endemic countries⁶⁰ and due to poor diagnostic and surveillance capacity this number is likely to represent an underestimate of the true burden of disease. Approximately 67,900 JE cases typically occur annually (overall incidence: 1.8 per 100,000) and recent data show that the global incidence of JE remains substantial despite improvements in vaccination coverage, especially young children in endemic areas are most susceptible.⁶⁰ As vaccination is the most feasible, reliable and cost effective tool for JE control, IXIARO® with confirmed excellent safety profile is highly recommendable, in particular for vaccination of children at risk.

Several cell culture-derived JE vaccines from various producers are being developed and some of these vaccines have been licensed recently (Biken, Kaketsuken, Intercell). Jebik V produced by Biken is available only for the local market, JeimmugenInj from Kaketsuken is marketed in South Korea.

Discussion

Well established safety and tolerability

IXIARO® is a purified, inactivated aluminum-adjuvanted JE vaccine, based on the SA14–14–2 virus strain, and is available in North America, Europe Canada, Switzerland, Singapore, Hong Kong and Israel as well as in Australia & New Zealand (as JESPECT®). It is presented as a ready-to-use suspension for injection in a single-dose pre-filled syringe. The vaccine is produced from a well established Vero cell line, thus avoiding the potential serious adverse reactions that have been associated with the application of mouse-brain-derived products, and does neither contain stabilizers, nor preservatives. Two intramuscular immunizations of the vaccine given 4 weeks apart are intended to achieve optimal protection against JE. The Advisory Committee on Immunization Practices (ACIP) stated that Vero cell culture derived JE vaccine is a promising development for humans given its favorable immunogenicity and reactogenicity profile after a 2-dose primary series. In addition, because it does not contain gelatin or murine proteins, it might be associated with fewer hypersensitivityor neurological adverse events (AEs) than the mouse brain-derived vaccine.¹³

The safety, tolerability and immunogenicity profile of IXIARO® is well established through a number of clinical studies comparing IXIARO® with a placebo control vaccine (Alum-containing PBS) as well as mouse brain derived vaccine.^36,46,53 A comprehensive review about the safety profile of IXIARO® in the first 12 months after licensure was published in 2011 ⁶¹ and discussed the observed profile in the context of clinical trial results for IXIARO® and post-marketing safety data for JE-VAX® as reported to VAERS between 1999 and 2009. The clinical safety profile was derived from a pooled analysis including safety data from 10 phase 3 trials in 4043 subjects who received at least one IXIARO® vaccination and were followed-up for up to 3 y after the primary immunization. Local and systemic tolerability of IXIARO® was similar to an earlier safety analysis at the time of licensure of the vaccine.

In post-marketing AE reports, the system organ classes affected following vaccination with IXIARO® were similar to the previously observed clinical trial profile. The number of IXIARO® doses distributed in Europe, USA, and Australia (trade name: JESPECT®) was derived from 2 distribution partners and Valneva. Used IXIARO® doses were estimated based on the number of distributed doses. The AE reporting rate was derived from the total number of AEs per predefined time period and per 100,000 doses distributed. The serious AE reporting rate was the total number of AEs per predefined time period and per 100,000 doses distributed. Hypersensitivity and neurological AEs (considered AEs of special interest) were classified based on the Brighton Collaboration. ⁶²All AE reports received during 12 months after international launch of IXIARO® (i.e., from 01 April 2009 to 31 March 2010) were reviewed and included in this analysis. No serious allergic reactions were observed in the 12-month post-marketing period. Hypersensitivity was reported more frequently with JE-VAX® with a rate of 8.4/100,000 doses compared to 3.6 for IXIARO®. Paraesthesia and injection site reactions were also both more common with JE-VAX®. The 5 most common adverse events with JE-VAX®, rash, pruritis, urticaria, headache, and fever, were all reported at lower rates per 100,000 doses administered for IXIARO®, except for fever were rates were similar.

This long-term safety data confirmed the good safety profile of IXIARO® in clinical and post-marketing use in a large population, consistent with that determined from the clinical trial results. In addition, irrespective of causality, the low post-marketing serious ADR reporting rate of only 1.6/100,000 doses distributed was also consistent with the low rate of serious AEs (2% of all subjects) in clinical trials.

A further review of postmarketing safety data included all ADRs reported within 36 months of licensure.⁶³ The overall ADR reporting rate was 13.0 / 100,000 doses, clearly within the range to be expected for vaccines in general, which is 2–30 cases / 100,000 doses; ^64,65 the serious ADR reporting rate was 1.6 / 100,000 doses. The safety profile remained favorable and consistent with clinical trials.

Nevertheless it is worth to mention that the vaccines produced in Japan (Jebik V by Biken, JeimmugenInj by Kaketsuken)^66,67 are, in contrast to IXIARO®, lyophilized, unadjuvanted Vero cell derived vaccines based on Beijing-1 strain with less antigen per dose. These inactivated cell culture-derived vaccines have also been shown to be safe and efficacious in humans.

Cross protection

All current JEV vaccines worldwide are prepared from genotype III strains and have shown to be effective against other locally circulating strains.⁶⁸ Since the 1990s there is a geodynamic shift in the predominant circulating JEV genotype from genotype III to genotype I not only in Japan, but also in China and India.⁶⁹ The ability of IXIARO® to protect against heterologous strains - both JEV SA14 (genotype III) and KE-093 (genotype I) - was demonstrated in a mouse JEV challenge model ⁴³ as well as in a 2-year follow-up study. ⁷⁰Two years after the primary vaccination series with IXIARO®, 87–93% of the vaccinees proved to be cross-protected against test strains representing genotypes II-IV and 73% against those of genotype I. After a single homologous or heterologous booster dose to JE-VAX® -primed subjects, the 2-year seroprotection rates against genotype I–IV strains were 89–100%. A second study in military personnel primed with mouse-brain derived JE vaccine also confirmed the ability of IXIARO to boost the immune system after priming with a heterologous inactivated whole-virus JE vaccine, with 100% of the subjects achieving seroconversion after one dose of IXIARO.⁷¹

Positive opinion from the EMA for vaccination of children

In endemic areas, children are the most vulnerable population for JE, a potentially debilitating or fatal disease for which there is currently no drug treatment available. As the mortality rates can go as high as 30% of symptomatic cases and about 50% of those who survive have permanent neurological sequale, there is a need for a safe and immunogenic vaccine in this vulnerable population (<18 years). In accordance with the approved Pediatric Investigation Plan (PIP) 3 clinical studies evaluating IXIARO® in children were performed. Studies to assess the systemic and local safety profile and immunogenicity of IXIARO® administered in 2 doses in a 28-day interval in a pediatric population (>2 months to 18 years) from endemic and from non-endemic regions have been conducted and are partly still ongoing. Pivotal data come from the Phase III trial in the Philippines, a JE-endemic area, involving 1869 children aged 2 months through to 17 y of age.⁷² In this randomized, active-controlled, open-label study, groups were selected to receive full (6 μg = 0.5 ml for ages 3 y and above) or half (3 μg, 0.25 ml for children below 3 y of age) doses of IXIARO® (2 doses 1 month apart), Havrix® hepatitis A vaccine or Prevnar® 7-valent pneumococcal conjugate vaccine. IXIARO® was well tolerated and immunogenic up to 6 months after the second dose. The safety profile was generally comparable with the age specific control vaccines – Prevnar in children younger than one year of age and Havrix in children aged ≥1 year. Seroconversion rates (SCR) 4 weeks after the second dose were high: >99% of children who received the age-appropriate dose achieved protective antibody titers (50% plaque reduction neutralization test titer ≥1 :10) and 77–100% of children achieved a ≥fourfold increase over their baseline titers.⁷³ An ongoing follow-up study in 300 children from this pediatric study population is evaluating long-term immunity with or without administration of a booster (third) dose given 12 months after the first dose of the primary series. A booster led to pronounced increase in PRNT titers (100% SCR and GMTs 890 – 4076 one month after booster, highest in youngest children). One year after the booster, titers in booster group remained above the cut-off defined for seroprotection (PRNT50≥1 :10) in all subjects (SPR 100%, GMTs 231 – 623 depending on age), and GMTs remained elevated compared to pre-booster GMTs 1 y after primary series (pre-booster GMTs 26 – 69). Titers also remained above the threshold for seroprotection without a booster in a substantial proportion of that group (SPR 81 - 100%, GMTs 39 – 92) at Month 24. The booster was well tolerated, with no apparent long term Adverse Events.⁷⁴

In addition, data in 100 children from the parallel Phase III ongoing pediatric study in children from JE-nonendemic regions, mostly in Europe, who are planning to travel to JE-endemic regions, confirmed that IXIARO® is tolerable and immunogenic in a nonendemic pediatric population, with a 100% SCR.⁷⁵

As the overall benefit risk ratio of IXIARO® in adolescents and children from 2 months onwards is positive, the European Commission approved the extension of indication of IXIARO® to the pediatric segment (2 months of age and older) based on these data and following a positive opinion from the EMA.

Following the FDA approval in May 2013, the Advisory Committee on Immunization Practices (ACIP) voted in June 2013, to extend existing recommendations for use of the vaccine IXIARO® and to include children aged 2 months through 16 years,⁷⁶ the recommendations for use of JE - vaccines for the primary series in children aged 2 months through 16 y are the same as for persons aged ≥17 years traveling to JE-endemic countries.

 

Table 1. Summary of trials (clinicaltrials.gov)

Study	Overview of Study Design	Treatment	Dose/regimen	No Subjects
WRAIR 763	Randomized, dose and schedule-finding, phase I	JE-PIV	0.4μg/Days 0 and 28 0.4μg, Days 0, 7 and 28 2μg, Days 0 and 28 2μg, Days 0, 7 and 28	25
WRAIR 815	Randomized, active-controlled (JE-VAX), open-label, dose and schedule-finding, phase II	JE-PIV JE-VAX	6μg, Days 0 and 28 6μg, Days 0, 14 and 28 12μg, Days 0 and 28	94
IC51–301 Pivotal Efficacy Study	Randomized, active-controlled (JE-VAX), observerblind, non-inferiority, phase III	IC51 JE-VAX	6μg, Days 0 and 28	867
IC51–302 Pivotal Safety Study	Randomized, double-blind, placebo-controlled multicenter phase III	IC51 Placebo	6μg, Days 0 and 28	2675
IC51–303	Multicenter, follow-up phase III	Not applicable (IC51 or JE-VAX in IC51–301 and 302)	Not applicable	3258
IC51–304	Randomized, observer-blind, non-inferiority (1x12μg vs. 2x6μg vs. 1x6μg), phase III	IC51	6μg, Days 0 and 28 6μg, Day 0 12μg (2x6μg), Day 0	374
IC51–305	Open-label, follow-up, phase III (booster given at month 11 a/o 23 if PRNT negative)	IC51	6μg, Month 11 and/or 23	349
IC51–308	Randomized, active-controlled, single-blind, non-inferiority (IC51+HAVRIX vs. IC51 vs. HAVRIX), phase III	IC51 HAVRIX	6μg, Days 0 and 28	192
IC51–309	Randomized, active-controlled, double-blind, equivalence (of 3 clinical IC51 batches), phase III	IC51	6μg, Days 0 and 28	639
IC51–310	Randomized, active-controlled, double-blind, equivalence (of 3 commercial IC51 batches), phase III	IC51	6μg, Days 0 and 28	389
IC51–311	Open-label, follow-up, phase III (booster given at month 15), phase III	IC51	6μg, Month 15	198
IC51–314	Open-label, sequential cohorts study (immunogenicity at end of shelf-life), phase III	IC51	2x6μg	304
IC51–315	Open-label, single-arm safety and immunogenicity in Elderly, phase IV	IC51	2x6μg	200
IC51–221 (pediatric)	Randomized, active-controlled (JenceVac), observerblind, non-inferiority, children between 1 and 3 y of age, phase II	IC51 JenceVac	3μg or 6μg, Days 0 and 28	60
IC51–323 (pediatric) Pivotal Safety and Immunogenicity Study	Randomized, active-controlled, open label, children aged 2 months to 17 y in JE-endemic area, phase III	IXIARO HAVRIX Prevnar	3μg or 6μg, Days 0 and 28	1869
IC51–322 (pediatric)	Uncontrolled, open-label Immunogenicity and safety study in children aged 2-month to less than 18 y in non-endemic phase III	IXIARO	3μg or 6μg, Days 0 and 28	100
IC51–324 (pediatric)	Multicenter, follow-up phase III	Not applicable	Not applicable	ongoing
IC51–325 (pediatric)	Multicenter, follow-up phase III	IXIARO	3μg or 6μg, Month 12	300

Disclosure of Potential Conflicts of Interest

BJ served as a consultant in the planning of various trials as well as a member of DSMBs.