Immunogenicity and safety of the tick-borne encephalitis vaccination (2009-2019): A systematic review

: BACKGROUND Tick-borne encephalitis (TBE) is increasing in Europe. We aimed to evaluate the immunogenicity and safety of TBE-vaccination. METHODS This systematic review was registered at PROSPERO (CRD42020155737) and conducted in accordance with PRISMA guidelines. We searched CINAHL, Cochrane, Embase, PubMed, and Scopus using specific terms. Original articles, case reports and research abstracts in English, French, German and Italian were included for screening and extracting (JER; PS). RESULTS Of a total of 2464 records, 49 original research publications were evaluated for immunogenicity and safety. TBE-vaccines showed adequate immunogenicity, good safety and interchangeability in adults and children with some differences in long-term protection (Seropositivity in 90.6-100% after primary vaccination; 84.9%-99.4% at 5 year follow up). Primary conventional vaccination schedule (days 0, 28, and 300) demonstrated the best immunogenic results (99-100% of seropositivity). Mixed brand primary vaccination presented adequate safety and immunogenicity with some exceptions. After booster follow-ups, accelerated conventional and rapid vaccination schedules were shown to be comparable in terms of immunogenicity and safety. First booster vaccinations five years after primary vaccination were protective in adults aged <50 years, leading to protective antibody levels from at least 5 years up to 10 years after booster vaccination. In older vaccinees, > 50 years, lower protective antibody titers were found. Allergic individuals showed an adequate response and immunosuppressed individuals a diminished response to TBE-vaccination. CONCLUSIONS The TBE-vaccination is generally safe with rare serious adverse events. Schedules should, if possible, use the same vaccine brand (non-mixed). TBE-vaccines are immunogenic in terms of antibody response but less so when vaccination is started after the age of 50 years. Age at priming is a key factor in the duration of protection. A S Background: Tick-borne encephalitis (TBE) is increasing in Europe. We aimed to evaluate the immunogenicity and safety of TBE-vaccination. Methods: This systematic review was registered at PROSPERO (#CRD42020155737) and conducted in accor- dance with PRISMA guidelines. We searched CINAHL, Cochrane, Embase, PubMed, and Scopus using specific terms. Original articles, case reports and research abstracts in English, French, German and Italian were included for screening and extracting (JER; PS). Results: Of a total of 2464 records, 49 original research publications were evaluated for immunogenicity and safety. TBE-vaccines showed adequate immunogenicity, good safety and interchangeability in adults and children with some differences in long-term protection (Seropositivity in 90.6 – 100% after primary vaccination; 84.9% – 99.4% at 5 year follow up). Primary conventional vaccination schedule (days 0, 28, and 300) demonstrated the best immunogenic results (99 – 100% of seropositivity). Mixed brand primary vaccination presented adequate safety and immunogenicity with some exceptions. After booster follow-ups, accelerated conventional and rapid vaccination schedules were shown to be comparable in terms of immunogenicity and safety. First booster vaccinations five years after primary vaccination were protective in adults aged < 50 years, leading to protective antibody levels from at least 5 years up to 10 years after booster vaccination. In older vaccinees, > 50 years, lower protective antibody titers were found. Allergic individuals showed an adequate response and immunosuppressed individuals a diminished response to TBE-vaccination. Conclusions: The TBE-vaccination is generally safe with rare serious adverse events. Schedules should, if possible, use the same vaccine brand (non-mixed). TBE-vaccines are immunogenic in terms of antibody response but less so when vaccination is started after the age of 50 years. Age at priming is a key factor in the duration of protection.


Introduction
Being endemic in 27 European countries with around 5 ′ 000-10 ′ 000 notified cases annually, tick-borne encephalitis (TBE) is one of the most important causes of viral encephalitis and the most frequent cause of viral meningitis in Europe [1][2][3]. TBE is geographically focused in Central and Eastern Europe, the Baltic States, the Russian Federation, and Japan, trending towards both an expansion of risk areas and an increase in incidence [2][3][4][5][6][7]. In Switzerland, incidence of TBE has increased significantly in the last few years, with more than 350 cases recorded in 2018 [8].
TBE is caused by the human pathogenic TBE virus, which is a member of the Flaviviridae family [3,4,9,10]. Three subtypes based on geographic origin and antigenic characteristics are of human importance: Far-Eastern, Siberian, and European [4,11]. Most European TBE cases are tick-transmitted by the ticks Ixodes ricinus with more than 100 species of wild and domestic animals acting as hosts reservoir [9,12,13]. Additionally, in certain areas TBE cases are transmitted from ingesting unpasteurized milk or milk products of infected animals the so-called "alimentary TBE" and by Dermacentor reticulatus ricks respectively [10,14].
To describe a vaccine's potential to prevent an infection, the two terms efficacy and effectiveness are used. While efficacy describes the effect measured in clinical trials (i.e. under ideal circumstances), effectiveness represents the results based on an epidemiological investigation under real world circumstances [25]. For the European TBE vaccines an effectiveness of 95-99% has been calculated in Austrian field studies [2,24]. To measure the immunogenicity specific TBE antibodies are evaluated using different laboratory test methods as listed in Box 1. The neutralization test (NT) is the most reliable to compare the TBE-vaccines' immunogenicity [26,27]. These antibodies show an age dependency with decreasing levels in elderly while keeping the same avidity. This immune aging process is termed immunosenescence [3,9].

Aim
Using a systematic review, we aimed to evaluate safety and immunogenicity of TBE-vaccination.

Methods
We systematically reviewed original research papers addressing European TBE-vaccines' immunogenicity and safety in accordance with PRISMA guidelines [28]. The systematic review was registered at PROSPERO: #CRD42020155737.

Study eligibility and search strategy
To identify appropriate studies, the following international databases were systematically searched with specific search terms as shown in Appendix 1: CINAHL, Cochrane, Embase, PubMed, and Scopus. Inclusion criteria were papers in English, French, German or Italian language, published in the period from January 1st, 2009, to August 31st, 2019, and being original articles, case reports or research abstracts. A Cochrane systematic review, published in 2009, summarizes important earlier findings, therefore, we decided not to include studies published earlier than 2009 [17]. Exclusion criteria were papers in other languages than the above mentioned and animal studies.

Data extraction
An evidence-table was created in Microsoft Word to extract the relevant data of original research (including population, intervention, control group, outcomes (PICO), study type, vaccines, laboratory analysis). To assess the methodological quality of the studies selected, we analyzed the strength of each study (original research and published abstracts) as displayed in Appendix 2.

Statistical analysis
Results of immunogenicity and safety for the TBE vaccines were investigated by two researchers (JER, PS) to conclude evidence-based recommendations in a narrative form. Different available laboratory

Box 1 Laboratory tests to measure anti-TBE-antibodies
Laboratory tests NT -Neutralization Test [26,27] Serum sample or dilution of antibodies is mixed with a viral suspen-sion on top of host cells. Reaction of antibodies lead to antibody-mediated neutralization of the virus and protection of the host cells. Dilution of the sera leads to a minimumprotective concentration: a titer ≥ 1:10 is assumed to be protective. ELISA -Enzyme-Linked Immunosorbent Assay [77,78] Serum sample or dilution of antibodies is mixed onto an assay with antibody binding sites. If binding takes place an enzyme mediated reaction takes place which can be recorded. An approach to standard-ise the results is to present them in Vienna international units/ml (VIEU/ml), whereas tests are used to quantify a vaccine's immunogenicity but this leads to difficulties in comparing study results. Therefore, in this systematic review a study's laboratory test methodology is labeled in Table 1 and  Table 2. If available, the results acquired by an NT were used for interpretation.

Results
After removing duplicates and screening, 55 papers were selected for full-text assessment from the investigated databases. Three additional publications were identified for full-text assessment through checking the included papers' reference lists. Title-, abstract-, and full-text screening was conducted by two researchers (JER, PS). Of the 58 fulltext assessed papers, 49 publications (40 pieces of original articles, five research abstracts of poster/oral sessions, three case reports and one case series) were investigated for immunogenicity and safety. Of the 40 original abstracts 26 showed external funding and/or sourcing, including 20 with connections to vaccine companies. Relevant data of published original articles were included into two comprehensive tables (Tables 1 and 2), while data from research abstracts and case reports/ series were presented in a narrative form only. Nine studies were excluded for qualitative analysis after full-text assessment and were used for background information if relevant: Three systematic reviews were found and were considered for background discussion [13,17,29]. A further five reviews/expert opinions and one original article did not include relevant information for our analysis [3,11,[30][31][32][33]. A PRISMA flow diagram (Fig. 1) demonstrates the selection process. (Table 1) 37 investigated original articles reported immunogenicity data. Fully vaccinated individuals regardless of the route of vaccination or delays in booster intervals were found to have an adequate immune response [10,18,21,[34][35][36][37][38][39][40]. Data on primary vaccination schedules are displayed in Table 3. Furthermore, the European licensed vaccine FSME-Immun also showed cross protection against Far Eastern and Siberian TBEV strain subtypes [4]. In adults with allergies compared to vaccinees with no allergies, higher antibody levels were found after TBE-vaccination [22]. High levels of protective antibodies do not guarantee prevention of TBE [41]. Vaccine failure numbers were low and were associated with a more severe illness, occurring more often in elderly [2,42,43]. Detailed data on immunogenicity are shown in Table 1.

Immunogenicity
The elderly have lower antibody levels with a diminishing immune response starting in individuals aged >60 years and even in individuals aged ≥50 years [9,22,44]. Most investigated vaccine failures occurred in individuals aged ≥50 years but failures also occurred in younger individuals [2,45]. Further, individuals ≥60 years with an extra priming dose reported no TBE-vaccine failure [2].
In children, aged 1-15 years, the vaccine formulas of Encepur® and FSME-Immun® lead to high immunogenicity after primary vaccination of 95.6% up to 100% and high long-term seropositivity up to 5 years after primary vaccination [16,19,36,[46][47][48]. There seems to be no age-related differences in the avidity and functional activity of antibodies induced by vaccination [2,49].

Interchangeability of TBE vaccines
For both adults and children TBE vaccines can be largely interchanged for primary and booster vaccination [18,37,38,46]. However, one study demonstrated a faster decrease in seropositivity in children receiving a mixed primary vaccination schedule (two doses of FSME-Immun® Junior followed by one dose Encepur® Children) [16].
1) Digital Object Identifier; 2) SAE = Serious adverse events; LR = Local reactions; SR = Systemic reactions; Systemic reactions were considered not to be related to the vaccination; 3) E.C. = Encepur® Children; F-I.J. = FSME-Immun® Junior; 4) Dose-finding study of FSME-Immun® Junior; 5) Fever at 2nd dose only reported being much lower than 1st dose. Fever showed age dependency; 6) FSME-IJ = FSME-Immun® Junior; Ence. C. = Encepur® Children; SAE = Serious adverse events; Both vaccines present well tolerance in children 1-11 years of age. A significant lower rate of injection site reaction was reported after vaccination with FSME-Immun® Junior compared to Encepur® Children. Close to equal were both vaccines in terms of systemic reactions and fever. Fever was reported more often in children aged 1-2 years compared to other age groups and injection site reaction was showing the lowest rate in this age group; 7) based on all reports received by the Swiss Federal Office of Public Health or the National Drug Pharmacovigilance Center ("Schweizerische Arzneimittelnebenwirkungszentrale); 8) In a passive reporting system, such as the ones investigated, milder events tend to be reported at a lower rate making numbers of SAE overrepresented. Incidence of serious adverse events reported to be 2.3 (95%CI: 1.4-3.5) per 100 ′ 000 distributed TBE-vaccine doses. Incidence of any adverse drug reactions for any kind of vaccine was described to be 2.7 per 100 ′ 000 distributed vaccine doses; 9) AE = adverse events; SAE = serious adverse events; 10) 298 children assessed for adverse events within seven days of third vaccination dose. No statistically significant differences between Encepur® Children and FSME-Immun® Junior for first and second vaccination reported; 11) SAE = serious adverse events; SAE were considered unrelated to the study vaccine by the authors and happened during the long follow-up time. Elective surgeries were not considered as SAE. During the study period four deaths occurred (two grade IV glioblastomas, one myocardial infarction and one suicide). As the suicide did not receive intervention it was not included into the safety analysis, therefore, only three deaths are included into SAE; 12) SC = subcutaneous; IM = Intramuscularly; SAE = serious adverse events; There was a significant lower local adverse event rate of redness, swelling and local pain in the intramuscularly route compared to the subcutaneous; 13) ESPED -Erhebungseinheit für seltene pädiatrische Erkrankungen in Deutschland (German pediatric surveillance unit); 14) Half the anaphylaxis cases following unspecific vaccinations occurred after the first dose. Authors conclude that either another component in the vaccine was the origin of the anaphylaxis or another molecular pathway without need of sensitization started the anaphylaxis; 15) SIT = specific immunotherapy; In the group with specific immunotherapy females showed an equal frequency on adverse events compared to males, whereas females in the group without specific immunotherapy and in the healthy control group showed higher adverse events rate than men in the same groups. (Table 2) 17 original articles reported safety data. Local reactions/mild adverse events such as pain at the injection site, tenderness or local swelling were described in 24.8% (4.3-54%) of study participants [18,19,35,37,38,40,46,53,56,57]. Systemic reactions were reported in about 30% (0.6-45.9%) of vaccinees [18,35,38,40,56]. Fever was reported in 3.4% (0-9.7%) of vaccinees [18,19,35,38,40,46,47]. Systemic reactions were reported to be lower after the 2nd dose compared to the first dose administration [19]. Higher rates of local and systemic reactions were reported in 7-11 year old children compared to 1-2 and 3-6 year old age groups [18]. In adults, no age pattern of adverse events was found. Furthermore, the application route led to differences in adverse event reporting: A significantly lower local adverse event rate of redness, swelling and local pain in the intramuscular administration group compared to the subcutaneous group was reported. Systemic reactions were reported to be increased in the intramuscular group, however, this was not statistically significant [35].

Safety
Ten studies in our analysis comprising 4455 vaccinees reported no serious adverse events (SAE) [18,19,21,35,37,38,46,47,53,54]. Three studies described SAE: One Encepur® booster five-year follow-up study reported an incidence rate of 5% in 278 adults. These SAE were considered by the authors to be "life events" during the long follow-up and not related to the vaccination (including two grade IV glioblastomas and one myocardial infarction), the possibility of an etiologic link was suggested by Strojnik in 2017 describing neurotropic viral genome in glioblastoma cells [40,58]. The second study reporting SAE was a surveillance study in a passive Swiss reporting system and it described 19 SAE after unspecified TBE-vaccine administration, leading to a calculation of an incidence rate of 2.3 SAE in 100 ′ 000 distributed doses of vaccine [59]. The third publication, a retrospective analysis of a German pediatric surveillance database, presented two cases of anaphylactic shocks after TBE vaccination (one unspecified vaccine, one based on K23 -probably Encepur®). Based on TBE vaccines  [60].
Based on the data it wasn't possible to identify sex patterns of adverse events. Although one paper showed adverse events to be reported at a higher rate in healthy females and in allergic females without specific immunotherapy compared to healthy men and allergic men without specific immunotherapy [56]. Further data about safety is displayed in Table 2.

Immunogenicity and safety data
Four research abstracts of poster-/oral sessions and one case series reported data on immunogenicity in thymectomized children (presented in 2009) or juvenile idiopathic arthritis (JIA) patients (presented in 2015) [61,62]. An adequate response was achieved in these groups after full vaccination. In a cohort study of 33 adults a lower antibody response was found in individuals aged 60-80 years compared to age group 21-31 years (presented in 2012) [63]. Another controlled cohort study demonstrated an adequate protective antibody level after primary TBE vaccination in elderly [64]. One case-series described four reported vaccine failures: one patient was deemed not to be a vaccine failure case (no second booster vaccination), two individuals to be probable vaccine failures and one case to be a confirmed vaccine failure [24].
Three case reports and one case series reported safety data. Jiménez et al. described the use of a statistical measure, the Information Compound (IC) measure of association. An IC score of 3.0 was found for TBE vaccines suggesting a statistical association between TBE vaccine and facial paralysis, compared to an IC score of 3.1 for a H1N1 influenza pandemic vaccine, an IC score of 3.0 for a hepatitis b/a vaccine or an IC score of 2.3 for a yellow fever vaccine [65]. Another case report described the reactivation of immune thrombocytopenic purpura by a TBE vaccination (FSME-Immun®) with subsequent recovery [66]. A 3-case series investigated excessive daytime sleepiness and narcolepsy-cataplexy starting a few weeks, one month, and two months after TBE vaccination (vaccine unspecified) [67]. In an expert opinion forum, a case of a 2 year old-child with facial paralysis presenting two days after second TBE dose was considered to be unrelated to the TBE-vaccination [68].

Discussion
Our systematic review found TBE vaccines Encepur® and FSME-Immun® to be highly immunogenic, well tolerated and in all studies except one to be interchangeable. There were some conflicting results with regard to age at first vaccination and booster intervals and the timing of vaccine administration and the use of accelerated schedules ( Table 3). The immunogenicity of these vaccines has been shown to be adequate after primary vaccination and following booster doses in all age groups. The duration of seropositivity in individuals aged ≥50 years was reduced and studies point to reduced long-term protection in older  [18] 28 days after TD a with 2x Ence. C. d + 1x FSME-I. J.® e 100% 3x FSME-Immun® Junior 100% Beran et al. [40] 5 years after TD a with Encepur® Conventional schedule f 100% Rapid schedule f 100% Accelerated schedule f 99% Aerssens et al. [57] ≥8 years after TD a with FSME-I. J.® e J age range 8-17 years g 51% Dorko et al. [39] 8 months h after TD a with FSME-I. J.® e 90.9% Pöllabauer et al. [21] 4 years after TD a with FSME-I. J.® e total (age 1-15 years) 93.7% 5 years after TD a with FSME-I. J.  adults. In terms of safety, the European, licensed vaccines were found to be well tolerated in both children (aged 1-17 years) and in adults, with local injection site reactions in 24.8% (4.3-54%) and systematic reactions in 30% (0.6-45.9%) of vaccinees. Vaccine related serious adverse events (SAE) were rare.
The conventional TBE vaccination schedule (0,28, 300 days) was superior to other schedules in the short-term only [31,46]. Studies show that long-term immunogenicity, after several booster vaccinations, was comparable regardless of the primary vaccination schedule [29,40]. Nevertheless, rapid vaccination schedules should be administered only in individuals requiring protection within a short timespan (such as travellers).
The interchangeability of the two European vaccines was shown in several publications except one from Wittermann et al. which showed a faster decline of antibody levels after a mixed primary vaccination schedule [16,18,37,38,46]. It appears that a mixed vaccine approach can be considered but is not optimal.
Many countries consider that the primary vaccination schedule protects for at least 3 years (Austria, Germany, Sweden), whereas in Switzerland the recommended first booster dose is ten years after the primary schedule [69]. The evidence from this systematic review supports an earlier first booster dose at 3-5 years in children and adults [16,21,51,52,70].
Subsequent booster intervals of at least 5 years in healthy adults were recommended in five studies and indeed, adequate post-booster protection from 5 years up to ten years for adults and/or children was confirmed [2,10,21,37,40,50]. Our results show a safe immunogenicity of TBE-vaccines for up to ten years after booster vaccination in healthy children (seropositivity at ten year follow-up: 90.3%) and adults below 60 years (seropositivity at ten year follow-up: 77.3%-94%) although lower immunogenicity was observed in adults > 50 years of age. Older individuals who have had a 4-dose primary schedule show longer duration of seropositivity after booster doses [2]. Therefore, to ensure protection of older people, recommendations should include a fourth vaccination during the primary schedule and shorter booster intervals [2,22,54].
Evidence on immunogenicity and safety of TBE vaccination in special risk groups remains scant. In a cohort of 70 allergic individuals an immune response after TBE-vaccination was comparable to healthy controls [56]. In studies with limited numbers, immunosuppressed patients showed a lower immune response compared to healthy individuals [54,71]. For thymectomized individuals the evidence shows only an early decreased immune response later approaching levels comparable to healthy controls [55,61]. Immunosuppressed groups must be informed of their high-risk status and should receive an extra dose of TBE-vaccine for primary vaccination regardless of age. There are research gaps: We found no studies documenting incidental TBE vaccine use in pregnant or breastfeeding women. There are few data on use of the vaccine in diabetic patients. Study results were rarely stratified by age and sex, although there are some indications that this is important. TBE vaccines have both shown to be well tolerated in children and adults with a lower rate of injection site reactions reported with FSME-Immun® Junior compared to Encepur® Children [17][18][19]. In 10 out of 13 investigated studies analyzing SAE in 4455 individuals no SAE were recorded [18,19,21,35,37,38,46,47,53,54]. In a 5-year follow up study, an incidence rate of 5% SAE was reported for 313 investigated individuals. These SAE were considered "life events" unrelated to the vaccine [40]. In a Swiss surveillance study of 73 adverse events in the years 1991-2001 following TBE-vaccination 19 presented to be SAE corresponding to a rate of 2.3 SAE per 100,000 distributed doses. This time span includes the application of the old mouse-brain derived TBE vaccines [59]. Another study of a German pediatric surveillance database described anaphylactic shock after TBE-vaccination and showed an SAE incidence of 0.69 (0.67-1.2) [1.0 (0.99-1.4)] per million TBE doses administered [60]. In summary, SAE associated with TBE vaccination are rare.
The issue of the timing and the frequency of booster doses is important: Swiss vaccine recommendations, issued by the Federal Office of Public Health, recommend administration of TBE-vaccine to all healthy individuals (>6 years old) in all areas except the cantons of Geneva and Ticino. The primary vaccination schedule should be administered, depending on the vaccine used, at months 0, 1 and 5-12. Thereafter booster vaccinations are recommended every 10 years in all age groups [8]. Swiss recommendations for booster vaccines differ from other countries' guidelines where boosters are recommended at earlier intervals [72][73][74] (Table 4).
Vaccination coverage of TBE vaccination is not actively monitored in Switzerland and therefore it is not possible to describe actual coverage, amount of used vaccines or field effectiveness of TBE-vaccines in the Swiss population. An unpublished report suggests a national TBE vaccination coverage of 9.5% for four TBE doses (personal communication Vasiliki B). In Austria Heinz et al. described a field effectiveness for regularly TBE-vaccinated individuals estimated to be around 99% under best case scenario and 96% under worst-case assumptions [34]. To increase coverage, Switzerland's rules for vaccination availability were adapted in 2015: certain cantons allowed community pharmacists with vaccination certification to administer specific vaccines, such as TBE-vaccine without prescription [75]. To expand coverage of TBE-vaccine, the Swiss army recommended voluntary TBE-vaccinations in young recruits, since 2007 [76]. Because service is only mandatory for Swiss men, there needs to be found another way to reach Swiss females and those who are not of Swiss nationality.
A strength of this Systematic Review is that it was conducted in accordance with PRISMA guidelines [28]. Five online databases were searched to include all the important publications and to summarize most important evidence for the European TBE-vaccines and the main results are highlighted in Table 5. Limitations of this systematic review were the different approaches of the included and investigated studies making outcomes hard to compare. Per example different laboratory tests used like Enzyme-linked Immunosorbent Assay (ELISA) and NT may not always be comparable. With regard to capturing SAE, most of the vaccine studies investigated, had a small sample size and were not powered to detect rare or SAE. Surveillance systems did identify SAE Table 5 Main findings from the systematic review.
reports. Many of the key studies were conducted directly or funded by TBE vaccine manufacturers. Future surveillance systems need to be strengthened to enable detection of very rare adverse events as well as TBE cases to allow finetuning of risk assessment. Additionally, more research must be done on sex differences in TBE vaccine response and booster intervals for individuals 50-59 years of age, impact of age at priming and on vaccine response in the immunocompromised. To further evaluate TBE vaccine recommendations, it is essential to continuously follow up all previously vaccinated TBE cases with respect to the number of doses and the time of vaccination. This information should be collated in a vaccination register to avoid memory or reporting biases.
In conclusion, TBE vaccination is generally safe with rare serious adverse events. Schedules should, if possible, use the same vaccine brand (non-mixed) and be age adjusted. TBE vaccines are immunogenic in terms of antibody response but less so when vaccination is started later than the age of 50 years. Age at priming is a key factor in the duration of protection.

Conflicts of interest
None of the authors have relevant conflicts of interest to declare.    [21] 2019 (87% at 10yfu 1) ) 1) yfu = years of follow-up.