Abstract
Tick-borne encephalitis (TBE) vaccination strategies to induce optimal seroprotection in children are under constant evaluation. This multi-center, randomized, controlled, phase III clinical study examined antibody persistence in children aged 1–11 y following two prospectively administered doses of either the FSME-IMMUN® Junior or Encepur Children® vaccines, as well as investigating the immunogenicity, safety and vaccine interchangeability of a third vaccination with FSME-IMMUN® Junior. A high level of antibody persistence was observed in all subjects 6 mo after the first of two vaccinations with either pediatric TBE vaccine. Based on both immunological tests and viral antigens used, slightly higher seropositivity rates and higher GMCs /GMTs were found in children vaccinated with FSME-IMMUN® Junior compared with those who received Encepur® Children. Seropositivity rates across all age strata combined six months after the first vaccination with FSME-IMMUN® 0.25 mL Junior were 95.1% as determined by Immunozym ELISA, 93.2% as determined by Enzygnost ELISA and 95.3% as determined by NT; compared with 62.6%, 80.5% and 91.0% respectively after vaccination with Encepur® Children. A third vaccination with FSME-IMMUN® Junior induced 100% seropositivity in both study groups and was well tolerated as demonstrated by the low rates of systemic and injection site reactions. Subjects who received either FSME-IMMUN Junior® or Encepur® Children vaccine for the first two vaccinations and FSME-IMMUN Junior® for the third showed a comparably strong immune response regardless of the previous TBE vaccine administered, demonstrating that two vaccinations with Encepur® Children can successfully be followed by a third vaccination with FSME-IMMUN Junior®.
Introduction
Tick-borne encephalitis (TBE) is a viral infection of the central nervous system which has been shown to result in serious neurological sequelae in up to 46% of patients and to be fatal in a small number of cases.Citation1,Citation2
Although TBE in children is generally considered a milder illness than among adults, there have been a number of severe cases reported in the pediatric population.Citation3-Citation6 Despite strong evidence that TBE vaccines protect children and adolescents against clinical TBE, vaccine formulations and immunization strategies capable of ensuring optimal protection of the pediatric population combined with an acceptable tolerability profile are under continuous evaluation.Citation7
In Europe, two pediatric TBE vaccines are currently available on the market: FSME-IMMUN® Junior (Baxter AG), which is licensed for use in children and adolescents aged 1–15 y, and Encepur® Children (Novartis Vaccines and Diagnostics GmbH and Co.), indicated for children aged 1–11 y. FSME-IMMUN® Junior is based on the TBE virus strain Neudoerfl whereas Encepur® Children is based on TBE strain Karlsruhe- K23. The antigenic components of both inactivated vaccines are highly homologous with only a few variable amino acid positions in the envelope (E) protein gene which do not affect functionally important sequences.Citation8,Citation9 According to the manufacturers’ recommendations, the conventional pediatric primary vaccination schedule consists of 3 doses of the vaccine: the first and second vaccinations administered one to three months apart, followed by the third vaccination 5–12 mo later for FSME-IMMUN® Junior and 9–12 mo later for Encepur® Children. A rapid immunization schedule is available for both vaccines; with FSME-IMMUN® Junior, the first two vaccinations are administered 14 d apart, followed by the third vaccination 5–12 mo later.
Due to the similarities between the virus strains and vaccination schedules, the interchangeability of the two vaccines has been considered. Results obtained from clinical studies have demonstrated that the replacement of one TBE vaccine by the other as a booster vaccination (after complete primary immunization) is certainly possible.Citation10 However, differences in the vaccines’ antigen amount, excipients and production process put the viability of this option into question. In particular, it has been hypothesized that a potentially inadequate immune response could be expected when the vaccine brand is changed during the course of primary vaccination. Despite this, data directly comparing the interchangeability of the two vaccines in randomized clinical trials are limited. A clinical study has previously been conducted on the primary vaccination series with either FSME-IMMUN® Junior or Encepur® Children for the first and second vaccination, and Encepur® Children administered as the third vaccination.Citation11,Citation12 In that study, titers obtained by NT following the first and second vaccination with Encepur® Children were higher than those obtained following administration with FSME-IMMUN® 0.25mL Junior. However, the results of the previously published part of the present study,Citation13 did not confirm the Wittermann findings demonstrating that higher immunological responses (NT and ELISA based) were induced after two vaccinations with FSME-IMMUN Junior® than with Encepur® Children.To allow for an estimation of the level of protection against TBE infection in subjects who have entered the tick season with an incomplete primary immunization (previously reported as the first part of the current study, where two vaccinations with either FSME-IMMUN® Junior or Encepur® Children were administered),Citation13 we present here the antibody persistence between the second and third doses of the primary vaccination course, as determined during the second part of this prospective study.
The current study also provides the opportunity to compare the immunogenicity and safety of FSME-IMMUN® Junior vaccine as the third vaccination (following two vaccinations with either FSME-IMMUN® Junior or Encepur® Children) with that previously reported for Encepur® Children.Citation11
Results
Demography
A total of 299 children aged 1 to 11 y who had previously received two vaccinations during the previously reported part of this prospective studyCitation13 with either FSME-IMMUN® Junior or Encepur® Children were eligible to receive a third vaccination with FSME-IMMUN® Junior in the second part of the present study; 298/299 received a third vaccination with FSME-IMMUN® Junior. The immunogenicity analyses included a total of 296/299 subjects for whom blood samples after the third vaccinations were available. As expected for a study where subjects were randomized using an electronic data capture system with a block size > 2, the demographic characteristics of all age strata were well balanced for age, gender, weight and height.
Antibody persistence following two vaccinations with either FSME-IMMUN® Junior or Encepur® Children
Evaluation of immunopersistence between the second and third vaccination (180 d after the first vaccination) was based on data obtained from 149 subjects vaccinated with FSME-IMMUN® Junior and 150 with Encepur® Children, for whom blood samples were available. Inclusion in the analysis of immunopersistence depended on the availability of immunogenicity measurements in all the three different serological assays. Results are presented by means of descriptive statistics.
There was a high level of antibody persistence 6 mo after the first of two vaccinations with either FSME-IMMUN® Junior or Encepur® Children as measured by neutralization test (NT, using the Neudoerfl strain) and ELISA (assays based on both the Neudoerfl and K23 strains). While the present study was designed to provide a descriptive comparison only and was not powered to obtain definitive differences in seropersistence between the two vaccines, higher seropositivity rates and geometric mean concentrations/titers (GMCs /GMTs) were observed among children who received FSME-IMMUN® Junior than those who received Encepur® Children, irrespective of the assays used ().
Table 1. Antibody persistence following two vaccinations with either FSME-IMMUN® Junior or Encepur® Children (180 d after the first vaccination)
Persistence of TBE antibodies as measured by seropositivity rates according to NT at 6 mo was shown in 95.3% and in 91.0% subjects who received FSME-IMMUN® Junior or Encepur® Children, respectively, in the three age groups combined. The NT antibody titer (GMT) in subjects who had received FSME-IMMUN® Junior was 50.9 in subjects aged 1 to 2 y, 34.7 in children aged 3 to 6 y and 33.5 in those aged 7 to 11 y. In subjects who received Encepur® Children, GMTs were 32.3, 21.0 and 22.4 in the three age groups, respectively.
Persistence of TBE antibodies 180 d after the first of two vaccinations with FSME-IMMUN® Junior, as measured by Immunozym ELISA, which uses the Neudoerfl viral strain was shown in 95.1% in the three age groups combined. A lower seropositivity rate (62.6%) was observed in subjects who received Encepur® Children. The geometric mean antibody concentration was substantially higher among children who had received FSME-IMMUN® Junior: 49.7 (GMC 729.0) in subjects aged 1 to 2 y, 28.2 (GMC 433.8) in subjects aged 3 to 6 y and 22.4 (GMC 380.6) in the oldest children compared with those vaccinated with Encepur® Children: 12.9 (GMC 237.6), 8.6 (GMC 141.8) and 7.8 (GMC 127.5) respectively.
When the Enzygnost ELISA, based on the K23 viral strain, was used, seropersistence was shown in 93.2% of subjects who received FSME-IMMUN® Junior and in 80.5% of subjects who received Encepur® Children, in the three age groups combined. Despite this ELISA using the strain on which the Encepur® Children vaccine is based, GMCs were also lower among those subjects vaccinated with Encepur® Children in all three age groups, compared with results obtained from subjects vaccinated with FSME-IMMUN® Junior.
Antibody response after the third vaccination with FSME-IMMUN® Junior
Immunogenicity was assessed one month after a third vaccination with FSME-IMMUN® Junior in all children (n = 296) who had available immunogenicity measurements. A third vaccination with FSME-IMMUN® Junior induced a strong immune response in all subjects regardless of whether FSME-IMMUN® Junior or Encepur® Children was administered as first and second vaccination.
Seropositivity rates were 100% in all age strata as measured by NT and both ELISAs. Overall, GMTs and GMCs 28 d after the third vaccination were comparable in subjects who had previously received FSME-IMMUN® Junior and Encepur® Children, with only slight differences observed within age strata ().
Table 2. Antibody response 28 d after a third vaccination with FSME-IMMUN® Junior
Safety analysis
Safety was assessed in 298 subjects aged 1 to 11 y (n = 99 subjects aged 1 to 2 y; n = 100 aged 3 to 6 y; n = 99 aged 7 to 11 y) who received a third vaccination with FSME-IMMUN® Junior. Safety data obtained after the first and second vaccinations with either FSME-IMMUN® Junior or Encepur® Children has been reported elsewhere.Citation13
No serious adverse reactions were reported during this part of the study (from day 56, i.e., 28 d after the second vaccination to day 388 i.e., 28 d after the third vaccination. There were no clinically relevant differences between the groups that received FSME-IMMUN Junior® or Encepur® Children for the first and second vaccinations. Overall, adverse reactions were predominantly mild; two subjects (one primed with FSME-IMMUN® Junior and one with Encepur® Children) experienced a severe injection site reaction after the third vaccination. The most frequently reported symptoms of injection site and systemic reactions after the third vaccination were injection site pain, tenderness and headache ().
Table 3. Proportion of subjects reporting specifically queried symptoms of non-serious local and systemic reactions following the third vaccination with FSME-IMMUN® Junior
In subjects aged 1 to 2 y, rates of systemic reactions, fever and local reactions within 7 d of the third vaccination were 1.0%, 3.0% and 2.0%, respectively. In subjects aged 3 to 6 y, rates of systemic reactions, fever and local reactions within 7 d of the third vaccination were 4.0%, 4.0% and 17.0%. In the oldest subjects (aged 7 to 11 y), rates of systemic reactions, fever and local reactions within 7 d of the third vaccination were 11.1%, 0.0% and 30.3%. The substantially higher systemic and local reaction rates in the oldest age group can partially be explained by the reduced ability of younger children to localize and formulate their discomfort.
Discussion
This study evaluated antibody persistence in children aged 1–11 y 6 mo after the first of two vaccinations with either the FSME-IMMUN® Junior and Encepur® Children vaccine (administered in the first part of this study, and reported elsewhere),Citation13 as well as investigating the immunogenicity, safety and vaccine interchangeability of a third vaccination with FSME-IMMUN® Junior. The study as a whole was powered to determine non-inferiority after the second vaccination. The results of study part presented here are shown as a descriptive comparison of seropersistence up to Day 180, as well immunogenicity and safety after the third vaccination.
One of the challenges to improving pediatric immunization strategies is to identify vaccine formulations and schedules capable of ensuring a strong immune response while maintaining an acceptable safety profile. The study part presented here demonstrates that a third vaccination with FSME-IMMUN® Junior following two doses of either the FSME-IMMUN® Junior or Encepur® Children vaccines produced very strong immune responses and was well tolerated in children aged 1 to 11 y as shown by low rates of systemic and injection site reactions after vaccination. In terms of the safety profile observed after the third vaccination with FSME-IMMUN® Junior, there were no clinically relevant differences between the groups that received FSME-IMMUN Junior® or Encepur® Children for the first and second vaccinations. Overall, the frequency of adverse reactions reported may be regarded as expected for an inactivated TBE vaccine with an aluminum adjuvant.Citation14
As higher immunological responses have already been demonstrated after the first and second vaccination with FSME-IMMUN® Junior than with Encepur® ChildrenCitation13,Citation15 the elevated level of antibody persistence between the second and third doses (6 mo after the first vaccination) observed among children who had previously received FSME-IMMUN® Junior was expected. Although both vaccines induced good immune responses, irrespective of the assays used (NT based on Neudoerfl and ELISAs based on Neudoerfl and K23 strains), higher seropositivity rates and GMT/GMCs were reported following two vaccinations with FSME-IMMUN® Junior, with maximum levels determined when immunogenicity was evaluated by ELISA based on the Neudoerfl strain. It should be noted that an NT based on the K23 strain contained in Encepur® Children vaccine could not be obtained for use in this study, which may be considered a limiting factor with respect to the immunological assessment of this vaccine. However, the Enzygnost ELISA, which is based on the K23 strain, was utilized in this study and data confirmed the lower seropersistence with the Encepur® Children vaccine compared with that obtained with FSME-IMMUN® Junior, as also shown with the tests based on the Neurörfl strain.. The high levels of TBE antibodies persisting 6 mo after the first vaccinations with Encepur® Children and FSME-IMMUN® Junior vaccines suggest that sufficient protection is available against TBE during the first tick season following the start of a child’s three dose primary vaccination series.
However, a different randomized clinical trial directly comparing FSME-IMMUN® Junior and Encepur® Children in similar conditions (children aged 1–11 y received either FSME-IMMUN® Junior or Encepur® Children for the first two primary vaccinations and Encepur® Children for the third) reported a higher immunological response to Encepur® Children.Citation11,Citation12 Several explanations might be adduced to account for the higher immunological potential of FSME-IMMUN® Junior based on the present study: there is more TBE virus antigen (1.2 µg) present in the FSME-IMMUN® Junior vaccine formulation compared with that of the Encepur® Children vaccine (0.75 µg); the possible presence of an aggregated fraction of human serum albumin and TBE antigen particles in the FSME-IMMUN® Junior vaccine may enhance immune response;Citation11 although the antigenic components of both vaccines are highly homologous, the deduced amino acid variation identified at three positions of the E protein sequence of the Neudoerfl and K23 strains could potentially induce distinction in the antigenicity of the vaccine strains.Citation9,Citation12,Citation16The ability of even single amino acid exchanges to induce variability in strain virulence has been shown in a mouse model.Citation16 However, the extent to which all these factors are involved in the post-vaccination immunological outcome has, to our knowledge, never been studied. Therefore, the magnitude of their contribution to the different immunological responses to FSME-IMMUN® Junior and Encepur® Children remains unanswered.
Of particular clinical significance is the fact that all subjects who received either FSME-IMMUN® Junior or Encepur® vaccine for the first two vaccinations and FSME-IMMUN® Junior for the third showed a comparably strong immune response, regardless of the previous TBE virus vaccine administered, demonstrating that two vaccinations with Encepur® Children can be successfully followed by a third vaccination with FSME-IMMUN® Junior. In particular, concerns about a potentially inadequate immune response caused by changing vaccine brand during the course of primary vaccination were not substantiated.
Methods
Study design
A single-blind, multi-center, randomized, controlled, Phase III clinical trial was conducted in children aged 1 - 11 y (stratified by age in three groups: 1 to 2 y ,Citation1 3 to 6 y Citation2 and 7 to 11 y of age Citation3). Subjects in each age group were randomly assigned at a ratio of 1:1 to receive either FSME-IMMUN® Junior or Encepur® Children for the first and second vaccinations and FSME-IMMUN® Junior only for the third vaccination according to a conventional schedule (0, 28 and 360 d).
The immunological responses and safety profiles following the first and second vaccination have been published elsewhere.Citation13,Citation15 Here we present the evaluation of antibody persistence between the second and third doses of the primary vaccination course (6 mo after the first vaccination with either FSME-IMMUN® Junior or Encepur® Children) as well as the immunogenicity and safety following the third vaccination (administered approximately one year after the first vaccination) with FSME-IMMUN® Junior.
All subjects received the vaccine intramuscularly into the musculus deltoideus of the upper arm or—depending on the developmental and nutritional status of the child—into the musculus vastus lateralis of the upper leg.
The study was performed according to ICH-GCP guidelines and the Declaration of Helsinki.Citation17,Citation18 Approvals by the responsible ethics committees, as well as written informed consent for all children from their parents/legal guardians were obtained.
Study vaccines
FSME-IMMUN® Junior (FSME-IMMUN 0.25 mL Junior, Baxter AG, commercial lot number 091021A) contains 1.2 µg of a formaldehyde-inactivated, sucrose gradient purified TBE virus (strain Neudoerfl) stabilized by human serum albumin. Encepur® Children (Encepur 0.25 mL Children, Novartis Vaccines and Diagnostics GmbH and Co., commercial lot number 091021A) contains 0.75 µg of inactivated TBE virus (strain Karlsruhe [K23]) stabilized by sucrose. Both vaccines are preservative-free and adsorbed to Al(OH)3 (with 0.15–0.2 mg Al3+ per dose).
Immunogenicity assessments
For serological analysis of TBE antibody response, blood samples were taken six months after the first vaccination, as well as approximately one month after the third vaccination. Immune response was determined using two quantitative ELISAs - Enzygnost® Anti-TBE/FSME/ETG-Virus (IgM, IgG) [Dade Behring, catalog number OQSW11 (lots 38728, 39108, 39625)] based on the Karlsruhe K23 strain, and Immunozym® FSME IgG [Progen, catalog number 7701010 (lots A08/042, A09045, A09072, A09073)] based on the Neudoerfl strain, as well as neutralization test (NT), which also used the Neudoerfl strain.Citation19 Both ELISAs were performed by external reference laboratories (Institute of Virology and Institute of Medical Chemistry, Medical University of Vienna, Austria). The NT assay was performed at Baxter’s Serological and Biochemical Control Laboratories, Quality Control Vaccines
Titers of neutralizing antibodies were determined as presented elsewhere.Citation20 ELISA values > 126 VIE U/mL obtained by the Immunozym® ELISA were considered positive.Citation21 The Enzygnost® ELISA categorized results >10.32 U/mL as positive. NT titers ≥ 10 were considered positive.
Safety assessments
Subjects in all three age strata were included in the safety analysis data set if they received the third vaccination with the FSME-IMMUN® 0.25 mL Junior. The subject’s parents/legal guardians received a subject diary for documentation of AEs for a total period of 6 d (including the day of vaccination). These diaries were returned to the investigator at the follow-up visit. Investigators recorded any AEs reported during study visits.
Safety was assessed by the occurrence of local and systemic reactions, as well as body temperature, as recorded in the diaries. Body temperature was measured orally (in children aged ≥ 3 y) or rectally (in infants and toddlers aged < 3 y) once every evening from vaccination until day 6 after vaccination. In case of fever, body temperature was to be measured every 4 to 8 h until it returned to normal.
Fever was rated according to the Common Toxicity Criteria.Citation22 Temperature values were analyzed according to the definitions and guidelines declared by the Brighton Collaboration Fever Working Group.Citation23
Statistical methods
Assuming a drop-out rate of less than 15%, approximately 300 subjects were planned to be enrolled in the study. The study was powered to determine non-inferiority after the second vaccination. The results of the study part presented here are shown as a descriptive comparison of seropersistence up to Day 180, as well immunogenicity and safety after the third vaccination.
Point estimates and exact 95% two-sided confidence intervals were calculated for the immunogenicity endpoints and for the three age groups. Antibody titers were assumed to be log-normal distributed. The analyses of immunogenicity endpoints were conducted in the modified intent-to-treat subject population.
Point estimates and 95% CIs were presented for all safety parameters. The proportion of subjects reporting specifically queried symptoms of local and systemic adverse reactions was also presented.
| Abbreviations: | ||
| CI | = | confidence interval |
| E | = | envelope protein |
| ELISA | = | enzyme linked immunosorbent assay |
| GM | = | geometric mean |
| GMC | = | geometric mean concentration |
| GMT | = | geometric mean titer |
| NT | = | neutralization test |
| TBE | = | tick-borne encephalitis |
Conflict of Interest
E.M. Poellabauer, B.G. Pavlova, A. Loew-Baselli, S. Fritsch, A. Geisberger, P.N. Barrett and H. J. Ehrlich are Baxter employees and have received Baxter stocks and stock options. R. Prymula and R. Angermayr have received consulting fees for the conduct of the present and other clinical studies from Baxter.
Acknowledgments
The authors wish to thank all the investigators (Johannnes Neugebauer, MD; Austria and Vladimir Nemec, MD; Drahoslava Lévová, MD; Vera Hvizd’alová, MD; Iva Madejová, MD; Irena Válova, MD; Jana Krausova MD; Pavel Kosina, MD; Vera Ryvolová MD; Czech Republic) who conducted this study. The authors wish to thank the Baxter Clinical Study Team: David Perry, Barbara Valenta-Singer, Jael Bosman, Peter Harmacek, Ulrike Langhammer-Augustin, Wolfgang Draxler, Karima Benamara, and Jennifer Doralt for their contribution to the success of this study.
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