Persistence of the immune response after vaccination with the Japanese encephalitis vaccine, IXIARO® in healthy adults: A five year follow-up study

The durability of vaccine-induced protection: an overview

INTRODUCTION

Current vaccines vary widely in both their efficacy against infection and disease, and the durability of the efficacy. Some vaccines provide practically lifelong protection with a single dose, while others provide only limited protection following annual boosters. What variables make vaccine-induced immune responses last? Can breakthroughs in these factors and technologies help us produce vaccines with better protection and fewer doses? The durability of vaccine-induced protection is now a hot area in vaccinology research, especially after COVID-19 vaccines lost their luster. It has fueled discussion on the eventual utility of existing vaccines to society and bolstered the anti-vaxxer camp. To sustain public trust in vaccines, lasting vaccines must be developed.

AREAS COVERED

This review summarizes licensed vaccines' protection. It analyses immunological principles and vaccine and vaccinee parameters that determine longevity of antibodies. The review concludes with challenges and the way forward to improve vaccine durability.

EXPERT OPINION

Despite enormous advances, we still lack essential markers and reliable correlates of lasting protection. Most research has focused on humoral immune responses, but we must also focus on innate, mucosal, and cellular responses - their assessment, correlates, determinants, and novel adjuvants. Suitable vaccine designs and platforms for durable immunity must be found.

Safety and immunogenicity of a purified inactivated Zika virus vaccine candidate in adults primed with a Japanese encephalitis virus or yellow fever virus vaccine in the USA: a phase 1, randomised, double-blind, placebo-controlled clinical trial

BACKGROUND

Zika virus infection is a threat to at-risk populations, causing major birth defects and serious neurological complications. Development of a safe and efficacious Zika virus vaccine is, therefore, a global health priority. Assessment of heterologous flavivirus vaccination is important given co-circulation of Japanese encephalitis virus and yellow fever virus with Zika virus. We investigated the effect of priming flavivirus naive participants with a licensed flavivirus vaccine on the safety and immunogenicity of a purified inactivated Zika vaccine (ZPIV).

METHODS

This phase 1, placebo-controlled, double-blind trial was done at the Walter Reed Army Institute of Research Clinical Trials Center in Silver Spring, MD, USA. Eligible participants were healthy adults aged 18-49 years, with no detectable evidence of previous flavivirus exposure (by infection or vaccination), as measured by a microneutralisation assay. Individuals with serological evidence of HIV, hepatitis B, or hepatitis C infection were excluded, as were pregnant or breastfeeding women. Participants were recruited sequentially into one of three groups (1:1:1) to receive no primer, two doses of intramuscular Japanese encephalitis virus vaccine (IXIARO), or a single dose of subcutaneous yellow fever virus vaccine (YF-VAX). Within each group, participants were randomly assigned (4:1) to receive intramuscular ZPIV or placebo. Priming vaccinations were given 72-96 days before ZPIV. ZPIV was administered either two or three times, at days 0, 28, and 196-234. The primary outcome was occurrence of solicited systemic and local adverse events along with serious adverse events and adverse events of special interest. These data were analysed in all participants receiving at least one dose of ZPIV or placebo. Secondary outcomes included measurement of neutralizing antibody responses following ZPIV vaccination in all volunteers with available post-vaccination data. This trial is registered at ClinicalTrials.gov, NCT02963909.

FINDINGS

Between Nov 7, 2016, and Oct 30, 2018, 134 participants were assessed for eligibility. 21 did not meet inclusion criteria, 29 met exclusion criteria, and ten declined to participate. 75 participants were recruited and randomly assigned. 35 (47%) of 75 participants were male and 40 (53%) were female. 25 (33%) of 75 participants identified as Black or African American and 42 (56%) identified as White. These proportions and other baseline characteristics were similar between groups. There were no statistically significant differences in age, gender, race, or BMI between those who did and did not opt into the third dose. All participants received the planned priming IXIARO and YF-VAX vaccinations, but one participant who received YF-VAX dropped out before receipt of the first dose of ZPIV. 50 participants received a third dose of ZPIV or placebo, including 14 flavivirus-naive people, 17 people primed with Japanese encephalitis virus vaccine, and 19 participants primed with yellow fever vaccine. Vaccinations were well tolerated across groups. Pain at the injection site was the only adverse event reported more frequently in participants who received ZPIV than in those who received placebo (39 [65%] of 60 participants, 95% CI 51·6-76·9 who received ZPIV vs three [21·4%] of 14 who received placebo; 4·7-50·8; p=0·006). No patients had an adverse event of special interest or serious adverse event related to study treatment. At day 57, the flavivirus-naive volunteers had an 88% (63·6-98·5, 15 of 17) seroconversion rate (neutralising antibody titre ≥1:10) and geometric mean neutralising antibody titre (GMT) against Zika virus of 100·8 (39·7-255·7). In the Japanese encephalitis vaccine-primed group, the day 57 seroconversion rate was 31·6% (95% CI 12·6-56·6, six of 19) and GMT was 11·8 (6·1-22·8). Participants primed with YF-VAX had a seroconversion rate of 25% (95% CI 8·7-49·1, five of 20) and GMT of 6·6 (5·2-8·4). Humoral immune responses rose substantially following a third dose of ZPIV, with seroconversion rates of 100% (69·2-100; ten of ten), 92·9% (66·1-99·8; 13 of 14), and 60% (32·2-83·7, nine of 15) and GMTs of 511·5 (177·6-1473·6), 174·2 (51·6-587·6), and 79 (19·0-326·8) in the flavivirus naive, Japanese encephalitis vaccine-primed, and yellow fever vaccine-primed groups, respectively.

INTERPRETATION

We found ZPIV to be well tolerated in flavivirus naive and primed adults but that immunogenicity varied significantly according to antecedent flavivirus vaccination status. Immune bias towards the flavivirus antigen of initial exposure and the timing of vaccination may have impacted responses. A third ZPIV dose overcame much, but not all, of the discrepancy in immunogenicity. The results of this phase 1 clinical trial have implications for further evaluation of ZPIV's immunisation schedule and use of concomitant vaccinations.

FUNDING

Department of Defense, Defense Health Agency; National Institute of Allergy and Infectious Diseases; and Division of Microbiology and Infectious Disease.

Persistence of antibodies, boostability, and interchangeability of Japanese encephalitis vaccines: A systematic review and dose-response meta-analysis

BACKGROUND

The burden of Japanese encephalitis (JE) is substantial and is arguably one of the most serious viral encephalitic diseases with high case fatality and no specific treatment. JE vaccines are the only available mean to prevent the disease; however, the long-term persistence of antibodies, boostability, and interchangeability between different vaccine classes are not well understood.

METHODS

To summarise the evidence, PubMed, Embase, and Cochrane CENTRAL were systematically searched from their inception to March 2021. Dose-response meta-analysis was utilised to synthesise the proportion of individuals who were seropositive over time after a primary vaccination course and a booster dose. Proportion meta-analysis was conducted to estimate the proportion of individuals who were seropositive as well as those who reported adverse events following a booster dose with a different vaccine class.

RESULTS

Of 1053 publications retrieved, 27 studies with 4,558 participants were included. Of these, 11 studies assessed persistence of antibodies, 14 studies boostability, and 8 vaccine class interchangeability. The pooled seropositivity, 1-year after primary vaccination was 83.4% (95 %CI 78.2-89.5%) and remained stable for up to 5 years (82.7%; 95 %CI 76.1-89.4%). Rapid anamnestic response was observed 10 days post-booster dose, the proportion of individuals who were seropositive reached 96.9% (95 %CI 95.9-97.8%) and remained > 95% for up to 6 years. Inactivated mouse brain-derived vaccines followed by a booster dose of a different vaccine class was effective (i.e. seropositive 99%) and well tolerated.

CONCLUSIONS

A booster dose after the primary vaccination is effective and further booster doses may be needed after 7 years. Inactivated mouse brain-derived vaccine followed by a booster with a newer vaccine class is effective and safe; although, there is a paucity of data related to newer classes of vaccines interchangeability.

Vaccines

The safety of COVID-19 vaccines, as was the case last year, remains a large part of the focus in this volume. COVID-19 placed a large magnifying glass on both vaccines, specifically vaccine safety. This was most readily apparent as the number of records in VAERS ballooned to about 10 times the size from 2020 to 2021 (Vaccine Adverse Event Reporting System (VAERS), 2022) [S]. While we have added and/or improved VAERS during COVID-19, including adding or improving other vaccine safety surveillance tools like v-safe and vaccine safety datalink (Blumenthal, Phadke, et al., 2021) [MC], there is still room for improvement in these pharmacovigilance tools (Rizk et al., 2021) [r]. A major global initiative in this realm is the Global Vaccines Safety Blueprint 2.0 (GVSB2.0) (Organization, 2021, pp. 2021–2023) [S]. We wholeheartedly endorse these initiatives, which could significantly improve vaccine safety.

As noted in past SEDA issues, clinicians should be mindful of the risks of AEs and SAEs associated with each individual vaccine.

Structural and biochemical insights into flavivirus proteins

Flaviviruses are the fastest spreading arthropod-borne viruses that cause severe symptoms such as hepatitis, hemorrhagic fever, encephalitis, and congenital deformities. Nearly 40 % of the entire human population is at risk of flavivirus epidemics. Yet, effective vaccination is restricted only to a few flaviviruses such as yellow fever and Japanese encephalitis viruses, and most recently for select cases of dengue virus infections. Despite the global spread of dengue virus, and emergence of new threats such as Zika virus and a new genotype of Japanese encephalitis virus, insights into flavivirus targets for potentially broad-spectrum vaccination are limited. In this review article, we highlight biochemical and structural differences in flavivirus proteins critical for virus assembly and host interactions. A comparative sequence analysis of pH-responsive properties of viral structural proteins identifies trends in conservation of complementary acidic-basic character between interacting viral structural proteins. This is highly relevant to the understanding of pH-sensitive differences in virus assembly in organelles such as neutral ER and acidic Golgi. Surface residues in viral interfaces identified by structural approaches are shown to demonstrate partial conservation, further reinforcing virus-specificity in assembly and interactions with host proteins. A comparative analysis of epitope conservation in emerging flaviviruses identifies therapeutic antibody candidates that have potential as broad spectrum anti-virals, thus providing a path towards development of vaccines.

Vaccines

In this volume of the Side Effects of Drugs Annual, although other vaccines will be covered, the safety of COVID vaccines is the focus as COVID-19 has led to heightened attention on vaccine safety in general. As such, this chapter will be more relevant than ever before. As noted in past SEDA issues, clinicians should be mindful of the risks of AEs and SAEs associated with each vaccine.

Japanese Encephalitis Vaccines

Purpose of review

As an eminently vaccine-preventable disease, encephalitis caused by Japanese encephalitis virus (JEV) has attracted an unusually high degree of attention from those seeking to develop viral vaccines. Since the 1950s, all types of JEV vaccines including inactivated, recombinant and live attenuated ones have been licensed. As an example of an extremely successful endeavour, the time is ripe for reviewing the development of JEV vaccines and probing the reasons behind their uniform success.

Recent findings

Vaccines against JEV have come a long way since the first licensing in the mid-1950s of the mouse brain-grown-inactivated virus preparations, to the present day live-attenuated virus vaccines. A survey of the various inactivated and live vaccines developed against JEV provides a striking insight into the impressive safety and efficacy of all the vaccines available to prevent encephalitis from JEV. This review juxtaposes studies to understand naturally acquired immunity against JEV that have mostly been published post-2000, compares these with those elicited by vaccines and highlights the paucity of data on cell-mediated immune responses elicited by JEV vaccines.

Summary

This article not only seeks to make available the immense salient literature on this endeavour in one collection, but also queries the basis for the remarkable success of JEV vaccines, not least of which may be the ease of protecting against encephalitis caused by JEV. To conclude, the true test of the ingenuity of those dedicated to the pursuit of viral vaccines would be success against viral diseases such as HIV-AIDS and dengue that pose a far greater challenge to scientists.

Vaccination against Japanese encephalitis with IC51: systematic review on immunogenicity, duration of protection and safety

Japanese encephalitis is a disease caused by a flavivirus which is transmitted by mosquitos in endemic countries. Considering the potentially severe outcomes of the disease, vaccination is recommended for those at risk of exposure. During recent years, IC51 (IXIARO®, JESPECT®, JEVAL®) has increasingly been used to protect travellers from Europe and the USA. However, no systematic review exists that summarizes the currently available evidence on the immunogenicity and safety of this vaccine. We conducted a systematic review on the immunogenicity and safety of IC51, using the databases PubMed, MEDLINE, EMBASE and ClinicalTrials.gov (search date: 31 August 2019). Data extracted from included studies were grouped by outcomes and stratified by population and setting. Risk of bias (ROB) was assessed using the RoB 2 tool for randomized controlled trials (RCTs) and ROBINS-I for non-randomized studies. Due to high heterogeneity, meta-analysis was not performed. A total of 32 studies from 16 countries met the inclusion criteria (15 RCTs, 17 non-randomized studies). ROB was serious or high in the majority of studies. Seroprotection rates ranged from 93 to 100% in adults (seven studies) and from 91 to 100% in children (four studies). In the study involving adults aged 64 years and older, seroprotection was 65% with higher rates in persons who were previously vaccinated against tick-borne encephalitis virus. Safety was investigated in 27 studies. Rates of serious adverse events were below 5% in all age groups, with the majority not being causally related to the vaccine. IC51 is a safe vaccine with good seroprotective abilities in persons aged >2 months to <64 years. The body of evidence, however, is weakened by a large amount of heterogeneity in study and clinical trial methodology. Further well-designed RCTs with special risk groups are needed.

Comparative economic analysis of strategies for Japanese encephalitis vaccination of U.S. travelers

BACKGROUND

Japanese encephalitis (JE) virus is the leading vaccine-preventable cause of encephalitis in Asia. For most travelers, JE risk is very low but varies based on several factors, including travel duration, location, and activities. To aid public health officials, health care providers, and travelers evaluate the worth of administering/ receiving pre-travel JE vaccinations, we estimated the numbers-needed-to-treat to prevent a case and the cost-effectiveness ratios of JE vaccination for U.S. travelers in different risk categories.

METHODS

We used a decision tree model to estimate cost per case averted from a societal and traveler perspective for hypothetical cohorts of vaccinated and unvaccinated travelers. Risk Category I included travelers planning to spend ≥1 month in JE-endemic areas, Risk Category II were shorter-term (<1 month) travelers spending ≥20% of their time doing outdoor activities in rural areas, and Risk Category III were all remaining travelers. We performed sensitivity analyses including examining changes in cost-effectiveness with 10- and 100-fold increases in incidence and medical treatment costs.

RESULTS

The numbers-needed-to-treat to prevent a case and cost per case averted were approximately 0.7 million and $0.6 billion for Risk Category I, 1.6 million and $1.2 billion for Risk Category II, and 9.8 million and $7.6 billion for Risk Category III. Increases of 10-fold and 100-fold in disease incidence proportionately decreased cost-effectiveness ratios. Similar levels of increases in medical treatment costs resulted in negligible changes in cost-effectiveness ratios.

CONCLUSION

Numbers-needed-to-treat and cost-effectiveness ratios associated with preventing JE cases in U.S. travelers by vaccination varied greatly by risk category and disease incidence. While cost effectiveness ratios are not the sole rationale for decision-making regarding JE vaccination, the results presented here can aid in making such decisions under very different risk and cost scenarios.

Neutralizing antibody persistence in pediatric travelers from non-JE-endemic countries following vaccination with IXIARO® Japanese encephalitis vaccine: An uncontrolled, open-label phase 3 follow-up study

BACKGROUND

In an initial study among children from non-Japanese encephalitis (JE)-endemic countries, seroprotection rates remained high 6 months after completion of the primary series with IXIARO®.

METHODS

In this open-label follow-up study, a subset of 23 children who received a 2-dose primary series of IXIARO® in the parent study, were evaluated for safety and neutralizing antibody persistence for 36 months.

RESULTS

Seroprotection rates (SPRs) remained high but declined from 100% one month after primary immunization to 91.3% at month 7 and 89.5% at month 36. Geometric mean titers (GMTs) declined considerably from 384.1 by day 56-60.8 at month 36. No long-term safety concerns were identified.

CONCLUSIONS

The substantial decline in GMT observed in this study, together with previously published data on children vaccinated with IXIARO® support the recommendation for a booster dose in children who remain at risk of JE from 1 year after the primary series of IXIARO®, consistent with the recommendation for adults.

CLINICAL TRIAL REGISTRY NUMBER

NCT01246479.

Japanese Encephalitis Vaccine: Recommendations of the Advisory Committee on Immunization Practices

This report updates the 2010 recommendations from the CDC Advisory Committee on Immunization Practices (ACIP) regarding prevention of Japanese encephalitis (JE) among U.S. travelers and laboratory workers (Fischer M, Lindsey N, Staples JE, Hills S. Japanese encephalitis vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2010;59[No. RR-1]). The report summarizes the epidemiology of JE, describes the JE vaccine that is licensed and available in the United States, and provides recommendations for its use among travelers and laboratory workers.JE virus, a mosquitoborne flavivirus, is the most common vaccine-preventable cause of encephalitis in Asia. JE occurs throughout most of Asia and parts of the western Pacific. Approximately 20%-30% of patients die, and 30%-50% of survivors have neurologic, cognitive, or behavioral sequelae. No antiviral treatment is available.Inactivated Vero cell culture-derived JE vaccine (Ixiaro [JE-VC]) is the only JE vaccine that is licensed and available in the United States. In 2009, the U.S. Food and Drug Administration (FDA) licensed JE-VC for use in persons aged ≥17 years; in 2013, licensure was extended to include children aged ≥2 months.Most travelers to countries where the disease is endemic are at very low risk for JE. However, some travelers are at increased risk for infection on the basis of their travel plans. Factors that increase the risk for JE virus exposure include 1) traveling for a longer period; 2) travel during the JE virus transmission season; 3) spending time in rural areas; 4) participating in extensive outdoor activities; and 5) staying in accommodations without air conditioning, screens, or bed nets. All travelers to countries where JE is endemic should be advised to take precautions to avoid mosquito bites to reduce the risk for JE and other vectorborne diseases. For some persons who might be at increased risk for JE, the vaccine can further reduce the risk for infection. The decision about whether to vaccinate should be individualized and consider the 1) risks related to the specific travel itinerary, 2) likelihood of future travel to countries where JE is endemic, 3) high morbidity and mortality of JE, 4) availability of an effective vaccine, 5) possibility (but low probability) of serious adverse events after vaccination, and 6) the traveler's personal perception and tolerance of risk.JE vaccine is recommended for persons moving to a JE-endemic country to take up residence, longer-term (e.g., ≥1 month) travelers to JE-endemic areas, and frequent travelers to JE-endemic areas. JE vaccine also should be considered for shorter-term (e.g., <1 month) travelers with an increased risk for JE on the basis of planned travel duration, season, location, activities, and accommodations and for travelers to JE-endemic areas who are uncertain about their specific travel duration, destinations, or activities. JE vaccine is not recommended for travelers with very low-risk itineraries, such as shorter-term travel limited to urban areas or outside of a well-defined JE virus transmission season.