Preclinical investigations of the safety, immunogenicity and efficacy of a purified, inactivated tick-borne encephalitis vaccine

A systematic literature review of the effectiveness of tick-borne encephalitis vaccines in Europe

BACKGROUND

Tick-borne encephalitis (TBE) is an infectious disease caused by the tick-borne encephalitis virus (TBEV) in patients with symptoms of central nervous system (CNS) inflammation. More than 25 European countries have one or more TBE-endemic areas. Although two TBE vaccines, FSME-IMMUN® and Encepur®, are commonly used in Europe, there are no published reviews of the real-world effectiveness of TBE vaccines in Europe or elsewhere.

METHODS

We searched PubMed for TBE vaccine effectiveness (VE) articles and extracted information on country, study design, study period, study population, number of TBEV-infected cases, number of participants, and VE against TBEV infection and outcomes.

RESULTS

We identified 13 studies, conducted in Austria, the Czech Republic, Latvia, Germany, and Switzerland, published in 2003-2023. One study was a cohort investigation of a milk-borne outbreak. In the other studies, 11 (91.7%) used the screening method and two (16.7%) used a case-control design (one study used both). TBE vaccines were highly effective (VE estimates >92%) against TBEV infection in all age groups. Vaccines were also highly protective against mild infections (i.e., infections in patients without symptoms of CNS inflammation), and against infections resulting in TBE and hospitalization. Vaccines were also highly protective against the most serious outcomes such as hospitalization greater than 12 days. Product-specific VE estimates were also high, though limited data were available. Studies in Austria, the Czech Republic, Latvia, and Switzerland estimated that TBE vaccines prevented >1,000 TBE cases a year, avoiding many hospitalizations and deaths, in these countries combined.

CONCLUSIONS

Published VE studies demonstrate a high real-world effectiveness of the commercially available TBE vaccines in Europe. Although cases averted have been estimated in only four countries, TBE vaccination prevents thousands of cases in Europe each year. To prevent life-threatening TBE, TBE vaccine uptake and compliance with the vaccination schedule should be increased in residents of, and travelers to, TBE-endemic countries in Europe.

Tick-Borne Encephalitis Virus: A Quest for Better Vaccines against a Virus on the Rise

Tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, is one of the most important tick-transmitted viruses in Europe and Asia. Being a neurotropic virus, TBEV causes infection of the central nervous system, leading to various (permanent) neurological disorders summarized as tick-borne encephalitis (TBE). The incidence of TBE cases has increased due to the expansion of TBEV and its vectors. Since antiviral treatment is lacking, vaccination against TBEV is the most important protective measure. However, vaccination coverage is relatively low and immunogenicity of the currently available vaccines is limited, which may account for the vaccine failures that are observed. Understanding the TBEV-specific correlates of protection is of pivotal importance for developing novel and improved TBEV vaccines. For affording robust protection against infection and development of TBE, vaccines should induce both humoral and cellular immunity. In this review, the adaptive immunity induced upon TBEV infection and vaccination as well as novel approaches to produce improved TBEV vaccines are discussed.

Second five-year follow-up after a booster vaccination against tick-borne encephalitis following different primary vaccination schedules demonstrates at least 10 years antibody persistence

BACKGROUND

Tick borne encephalitis (TBE) endemic zones are expanding. We previously evaluated long term persistence of antibody 5 years after the first booster immunization following different primary immunization schedules with the polygeline-free inactivated TBE vaccine (TBEvac) in adults and adolescents. Here, we report anti-TBE virus (TBEV) antibody persistence from 6 to 10 years post-booster administration.

METHODS

This was a phase IV, open-label, single-center, second extension study (NCT01562444), conducted in Czechia. Healthy adults and adolescents ≥12 years who had received 3 different primary vaccination schedules (rapid, conventional and accelerated conventional) in the parent study and a booster dose before (12-18 months post-primary series completion) or at the beginning (3 years post-primary series completion) of the first extension study were screened and enrolled in this study. Blood samples were collected yearly and anti-TBEV antibody response was evaluated by neutralizing test (NT) antibody assays. Analysis was performed overall and per age strata: 15-49 years, ≥50 years, and ≥60 years.

RESULTS

Of 206 screened individuals, 191 completed the study. Overall, 90-100% of participants in the all-screened set and ≥97% in the per-protocol set had the clinically meaningful threshold of protection (NT titers ≥10) across all timepoints, regardless of the primary vaccination schedule. Overall, antibody geometric mean titers (GMTs) varied from 134 to 343 in the all-screened set. Older age groups showed overall lower GMTs, although GMTs remained higher than NT titers ≥10 up to year 10 in all groups.

CONCLUSION

This study showed long-term persistence of anti-TBEV NT antibodies for up to 10 years after the first booster dose of TBEvac in all age groups, regardless of the primary vaccination schedule.

Tick-borne encephalitis vaccines: past and present

Vaccines to protect against tick-borne encephalitis (TBE) are produced by two manufacturers and are widely used in European and Asian countries, where TBE virus is endemic. General trends in vaccine development during recent decades and extensive postmarketing experience resulted in several modifications to their formulations and practical implications for use. Modifications were made to the production process, such as the change of the virus master bank from mouse brain to primary cells; to the excipients, especially the stabilizers and preservative; and to include formulations for children. Additionally, a rapid vaccination schedule has been developed for persons who require a fast onset of protection. Recent data from clinical studies and postmarketing surveillance indicate that both vaccines are safe, efficacious and interchangeable. Further (major) changes to formulation or alternative targets for vaccine development are not anticipated in the next 5 years. Recent serologic studies indicate that the persistence of protective immunity was longer than expected. Thus, recommendations for prolongation of TBE booster intervals have been made in several European countries, and a harmonization for booster recommendations is predicted within the European Union. Based on epidemiologic trends, the use of TBE vaccines will continue to increase in all age groups, including children.

Zoonotic encephalitides caused by arboviruses: transmission and epidemiology of alphaviruses and flaviviruses

In this review, we mainly focus on zoonotic encephalitides caused by arthropod-borne viruses (arboviruses) of the families Flaviviridae (genus Flavivirus) and Togaviridae (genus Alphavirus) that are important in both humans and domestic animals. Specifically, we will focus on alphaviruses (Eastern equine encephalitis virus, Western equine encephalitis virus, Venezuelan equine encephalitis virus) and flaviviruses (Japanese encephalitis virus and West Nile virus). Most of these viruses were originally found in tropical regions such as Africa and South America or in some regions in Asia. However, they have dispersed widely and currently cause diseases around the world. Global warming, increasing urbanization and population size in tropical regions, faster transportation and rapid spread of arthropod vectors contribute in continuous spreading of arboviruses into new geographic areas causing reemerging or resurging diseases. Most of the reemerging arboviruses also have emerged as zoonotic disease agents and created major public health issues and disease epidemics.

Tick-borne encephalitis (TBE) vaccination: Applying the most suitable vaccination schedule

Tick-borne encephalitis (TBE) is caused by an arthropod-borne virus, belonging to the family of Flaviviridae. In case of disease, which can lead to neurological sequelae or even fatal outcomes, only symptomatic treatment is available. TBE can be prevented by vaccination. Various primary immunization schedules have been developed. To identify the most suitable schedule, the present randomised, controlled study was designed to provide data on the immune response elicited by four different immunization schedules obtained by ELISA and by neutralization test (NT). A total of 398 healthy subjects aged > or =12 years were randomised to vaccination according to either the rapid schedule (Group R, vaccination on days 0, 7 and 21), the conventional schedule (Group C, vaccination on days 0, 28 and 300), the modified conventional schedule (Group M, vaccination on days 0, 21 and 300) or the accelerated conventional schedule (Group A, vaccination on days 0, 14 and 300). Within 3 weeks (i.e. by day 21) antibody levels were higher in Group R and Group A than in Group M and Group C. Group R and Group C both had higher titres on days 42, 180 and 300, than Group A and Group M. The rapid schedule thus combines the advantages of fast protection and of high titres over the observation period of 300 days.

Protection against tick-borne encephalitis with a new vaccine formulation free of protein-derived stabilizers

BACKGROUND

Vaccination against tick-borne encephalitis (TBE) has been successfully employed for many years in TBE-endemic countries. Post-marketing experience gained from widespread use, however, prompted the development of improved TBE vaccines, the most modern versions of which do not contain the commonly used protein-derived stabilizers (human albumin or polygeline) of former vaccines.

METHOD

This article summarizes both the medical need for and clinical experience with a new TBE vaccine formulation (pediatric and adult versions). To this end, data from clinical trials and post-marketing experience are presented. The clinical database comprises immunogenicity and/or safety data of approximately 7,500 subjects ages 1 to 77 years who participated in eight clinical trials. The clinical trials were conducted at 69 centers in five European countries. Post-marketing experience includes safety data from passive pharmacovigilance systems in 18 countries where these vaccines have been licensed since 2001.

RESULTS

All subjects analyzed for immunogenicity achieved postimmunization levels of TBE antibodies that meet the definition of seroconversion or represent a fourfold increase. The pooled data of clinical trials revealed the expected rate of solicited local and systemic reactions. The majority of these transient postimmunization reactions were mild. Pharmacovigilance data confirm the high level of safety of these new TBE vaccines: only a common range of the side effects already noted for licensed TBE vaccines was reported. After the distribution of more than five million vaccine doses, no potential safety risk was noted.

CONCLUSION

Post-marketing experience supports results from clinical trials showing that these new TBE vaccines may safely be used for the vaccination of children, adolescents, and adults.

Stimulation of the immune system by different TBE-virus vaccines

Tick-borne encephalitis (TBE) is one of the most frequent arthropod-transmitted viral diseases in Europe. Different vaccines against TBE-virus have been developed; a thimerosal-free and also albumin-free vaccine [Ticovac (Baxter Hyland Immuno, Vienna)] was approved in 2000. Contrary to previous experience, 779 cases of fever occurred following the first vaccination of children under 15 years of age and in 62 children febrile convulsions were even observed. Consequently, the composition of the vaccine was changed and albumin was again added [FSME-Immun (Baxter Vaccines, Vienna)] in 2001. The new Encepur Kinder (Chiron-Behring, Marburg) from 2002 is a TBE-vaccine for children without any protein as stabilizer but with a relatively high concentration of sucrose, while the former vaccine Encepur K from 1991 contained polygeline as the stabilizer. The induction of the immune system by the different TBE virus vaccines was compared in an in vitro test in order to find an explanation for the unexpected fever attacks. Whole blood was stimulated with complete vaccine suspension, and TNF-alpha, IL-1beta, IL-6, and IL-8 were determined from heparin/EDTA-plasma and culture supernatants. It was shown that Ticovac and the new Encepur Kinder can induce relatively high amounts of TNF-alpha and lower amounts of IL-1beta. An increase of both cytokines was first observed following an incubation of 4 hours, with a maximum after 15 hours. Concentrations returned to base-line values within 26 hours. The behaviour of both cytokines correlates with the febrile phases in children up to two years old. Albumin or other proteins like polygeline and also immunoglobulins prevented a rise of cytokines.

Vaccines and animal models for arboviral encephalitides

Arthropod-borne viruses ("arboviruses") cause significant human illness ranging from mild, asymptomatic infection to fatal encephalitis or hemorrhagic fever. The most significant arboviruses causing human illness belong to genera in three viral families, Togaviridae, Flaviviridae, and Bunyaviridae. These viruses represent a significant public health threat to many parts of the world, and, as evidenced by the recent introduction of the West Nile virus (WNV) to the Western Hemisphere, they can no longer be considered specific to any one country or region of the world. Like most viral diseases, there are no specific therapies for the arboviral encephalitides; therefore, effective vaccines remain the front line of defense for these diseases. With this in mind, the development of new, more effective vaccines and the appropriate animal models in which to test them become paramount. In fact, for many important arboviruses (e.g. California serogroup and St. Louis encephalitis viruses), there are currently no approved vaccines available for human use. For others, such as the alphaviruses, human vaccines are available only as Investigational New Drugs, and thus are not in widespread use. On the other hand, safe and effective vaccines against tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus (JEV) have been in use for decades. New challenges in vaccine development have been met with new technologies in vaccine research. Many of the newer vaccines are now being developed by recombinant DNA technology. For example, chimeric virus vaccines have been developed using infectious clone technology for many of the arboviruses including, WNV, JEV, and TBEV. Other successful approaches have involved the use of naked DNA encoding and subsequently expressing the desired protective epitopes. Naked DNA vaccines have been used for TBEV and JEV and are currently under development for use against WNV. The development of less expensive, more authentic animal models to evaluate new vaccines against arboviral diseases will become increasingly important as these new approaches in vaccine research are realized. This article reviews the current status of vaccines, both approved for use and those in developmental stages, against the major arboviral encephalitides causing human disease. In addition, research on animal models, both past and present, for these diseases are discussed.

Albumin is a necessary stabilizer of TBE-vaccine to avoid fever in children after vaccination

A thiomersal-free and also an albumin-free tick-borne encephalitis-vaccine (TBE-vaccine) was developed. This vaccine was approved by the Austrian health authorities in the year 2000. Contrary to previous experience, 779 cases of fever attacks occurred following the first vaccination of children under 15 years of age. The induction of the immune system by different TBE virus (TBEV) vaccines (FSME-Immun [1999], Ticovac [2000] and FSME-Immun [2001] all from Baxter Hyland Immuno, Vienna) was compared in an in vitro immune stimulation test in order to find an explanation for the unexpected fever attacks. It was shown that only Ticovac, which contains no albumin as a stabilizer, can induce relative high amounts of TNF-alpha (P < or = 0.0001) and lower amounts of IL-1 beta (P < or = 0.05). Increase of both cytokines is first observed following an incubation of 4 h. The maximum is reached after 15 h. After 26 h, it has reverted to the original value. The course of concentration of both cytokines corresponds to the time of observed febrile phases. Albumin or immunoglobulin prevents a rise of cytokines so that it is recommended to add the albumin again to the vaccine.

Vaccination against tick-borne encephalitis under therapeutic immunosuppression. Reduced efficacy in heart transplant recipients

Patients after organ transplantation are at an increased risk of microbial infections and might benefit from active vaccination. Due to therapeutic immunosuppression the efficacy of immunizations is, however, reduced and difficult to predict. Efficacy of vaccination against tick-borne encephalitis (TBE) using an abbreviated immunization schedule was compared in 31 heart transplant recipients (age: 54.5 +/- 11.5 years, mean time after transplantation: 53.5 +/- 23.7 months) under cyclosporine-based immunosuppression and 29 controls. TBE vaccination was well tolerated by the transplant recipients; spectrum and frequency of adverse events were similar to controls. In the transplant patients, seroconversion rate (35% versus 100%; p < 0.001) and the geometric mean of post-vaccinal antibody titres (0.98 (SF: 2.3) U/ml versus 5.46 (2.2) U/ml; p < 0.001) were markedly reduced in comparison to the control group. No clinical or demographic predictors of vaccination success could be established in the transplant patients. Due to the limited efficacy, TBE vaccination cannot be recommended as a routine procedure in heart transplant recipients at risk of TBE virus infection. TBE vaccination may be performed safely in selected cases, but repeated titre controls to confirm vaccination success would be required.

Defective adenoviruses as novel vaccines for the Flaviviridae

BACKGROUND

Vaccines against many flaviviruses, such as Japanese encephalitis virus (JEV), yellow fever virus (YFV) and tick-borne encephalitis virus (TBEV), have been successfully used for many years. Other diseases such as dengue fever (DF) and hepatitis C are still major public health problems as no licensed vaccines are in use.

OBJECTIVES

To review studies on the use of defective recombinant adenoviruses (Rads) as experimental flavivirus vaccines and comment on their use to prevent infections with other members of the Flaviviridae such as hepatitis C virus.

STUDY DESIGN

Recombinant adenoviruses, defective in their replication strategy, contain deletions in the E1 and E3 regions of the genome to increase the amount of foreign genetic material that can be inserted. The expression of foreign genes, inserted into these regions, can be driven by the adenovirus's own promoter, or by an additional viral promoters.

CONCLUSIONS

Rads have been successfully used to raise protective immunity in experimental models of infection with several viruses. They can elicit both humoral and cell-mediated immunity and can be given parenterally or by oral administration. In addition, their hepatotropism makes them suitable for tackling diseases such as hepatitis C. Careful design of the vaccine vectors is advised to ensure their efficacy and safety, and as hepatitis C is a persistent infection, it may be advisable to design Rads containing genes encoding for non-structural proteins in preference to structural proteins.

Recombinant and virion-derived soluble and particulate immunogens for vaccination against tick-borne encephalitis

Using different forms of the envelope glycoprotein E from tick-borne encephalitis virus we investigated the influence of physical and antigenic structure on the efficacy of vaccination. Different protein E-containing preparations were either derived from purified virions or were produced as recombinant proteins in COS cells. These included soluble dimeric forms (virion-derived protein E dimers with and without membrane anchor; recombinant protein E dimers without membrane anchor), micellar aggregates of protein E (rosettes), and recombinant subviral particles (RSPs). The structural differences between these immunogens were verified by sedimentation analysis, immunoblotting and epitope mapping with a panel of monoclonal antibodies. Specific immunogenicities were determined in mice in comparison to formalin-inactivated whole virus. Rosettes and RSPs were excellent immunogens and exhibited similar efficacies as inactivated virus in terms of antibody induction and protection against challenge, whereas all of the soluble forms were much less immunogenic. These data emphasize the importance of the immunogen's antigenic and physical structure for an effective stimulation of the immune system and indicate that RSPs represent an excellent candidate for a recombinant vaccine against tick-borne encephalitis.

Development of a candidate vaccine against Ross River virus infection

Ross River virus is a mosquito-borne alphavirus which causes several thousand cases of arthritis (epidemic polyarthritis) each year. In this study, binary ethylenimine (BEI) was used to destroy the infectivity of this virus without abolishing the antigenicity or immunogenicity of the virion. Mice immunized intramuscularly with BEI-inactivated virus, with or without Alhydrogel adjuvant, produced antibody which neutralized Ross River virus in vitro, and the mice also failed to develop viraemia when challenged intravenously with live virus. Serum neutralization and in vivo protection were greatest when BEI-inactivated virus was administered without adjuvant.

Rapid detection of viruses of the tick-borne encephalitis virus complex by RT-PCR of viral RNA

Studies were performed to identify a pair of primers, specific for the tick-borne encephalitis (TBE) virus complex of the Flaviviridae, with which to develop a rapid and specific identification system based on reverse transcription and the polymerase chain reaction (RT-PCR). The specificity of a putative primer pair was examined by RT-PCR of representative viruses from other antigenic complexes of the Flaviviridae and by computer sequence homology checks. All viruses of the TBE complex tested, with a single exception, were identified by RT-PCR using the identified primer pair. Accumulated data suggest that one of the putative primers identified in these studies may have flavivirus group specificity. The advantages of such a primer in the development of identification systems for all virus complexes of the Flaviviridae is discussed.

The molecular biology of tick-borne encephalitis virus. Review article

Tick-borne encephalitis (TBE) virus is a member of the flavivirus genus and the family Flaviviridae. Like other flaviviruses such as yellow fever, Japanese encephalitis or the dengue viruses, it is an important human pathogen, endemic in many European countries, Russia and China. The disease can be effectively prevented by vaccination with a formalin-inactivated whole virus vaccine. In recent years major advances have been made in the understanding of the molecular biology of TBE virus, including the complete sequence analysis of the genomic RNA of the European and Far Eastern strains. As shown in these studies, the virion RNA contains a single long open reading frame that codes for the structural proteins at the 5' end and the nonstructural proteins at the 3' end. Co- and posttranslational cleavages by a viral and cellular proteases lead to the formation of individual viral proteins. The mature virion is composed of an isometric capsid surrounded by a lipid envelope with two membrane-associated proteins. One of these, protein E, is of paramount importance for several important viral functions, especially during the entry phase of the viral life cycle. Protein E is also responsible for the induction of a protective immune response. A detailed map of antigenic sites has been established and the structure of an anchor-free form of E is currently being investigated by X-ray diffraction analysis. Understanding the molecular basis of the functions of this protein together with the knowledge of its three-dimensional structure may provide clues for developing specific antiviral agents. Protein E has also been shown to be an important determinant of virulence, with single amino acid substitutions at selected sites leading to attenuation. Engineering of such mutations into cDNA clones to produce new recombinant viruses may open up new avenues for the development of live vaccines.

Sequence of the NS 1 gene of the K 23 isolate of tick-borne encephalitis virus and identification of conserved motifs

The NS 1 gene from the K 23 isolate of tick-borne encephalitis virus has been sequenced. The NS 1 gene sequence, and many structural features of the NS 1 protein are heavily conserved. Six conserved peptides were also identified.

A randomized phase II study of a new tick-borne encephalitis vaccine using three different doses and two immunization regimens

A new, highly purified inactivated tick-borne encephalitis (TBE) vaccine (FSME-Vaccine Behring, BI 71.061) was recently registered in Germany. A multinational phase II study was performed in seven centres located in areas endemic for TBE. A total of 379 healthy adults were randomly allocated into three dosage groups (1.0, 1.5 and 2.0 micrograms antigen per dose, respectively) and into two immunization schedules [vaccination with one dose of 0.5 ml intramuscularly on days 0, 7 and 21 (abbreviated schedule), or on days 0, 28 and 300 (conventional schedule)]. Antibody response to vaccination was assayed by enzyme-linked immunosorbent assay (ELISA), haemagglutination inhibition test (HIT) and neutralization test (NT). Seroconversion rates in the different groups 28 days after one single dose were 75.3-83.5% in ELISA, 35.8-50.6% in HIT, and 100% in NT. All vaccinees showed seroconversion in all tests on day 42 in the conventional schedule and on day 35 in the abbreviated schedule, with the exception of one subject, who remained seronegative in HIT only. Geometric mean titres (GMT) of about 3000 in ELISA were achieved by two vaccinations in the conventional schedule and showed a booster increase to 5500-8000 GMT after revaccination on day 300. Overall frequency of adverse events (related and unrelated) was 37% (conventional schedule) and 46% (abbreviated schedule) after the first, 9% and 21% after the second, and 5% and 15% after the third vaccination, respectively. Generally, side effects were mild and transient, including mainly headache, fever, malaise and local irritation. Serious, vaccine-related side effects did not occur.(ABSTRACT TRUNCATED AT 250 WORDS)