The highest incidence of invasive disease due to Neisseria meningitidis in the United States is in infants below 1 year of age (5.38 cases per 100,000 population), and the peak incidence is in those under 8 months of age.1 Between 1998 and 2007, meningococcal serogroup B was responsible for 31% of meningococcal disease cases in all age groups and 58% of cases in children less than 1 year of age.1 Serogroups C and Y were together responsible for 62% of meningococcal disease, and for 38% of cases in children less than 1 year of age.1 In order to impact meningococcal disease in US infants and children, meningococcal conjugate vaccines need to target both serogroups C and Y at early ages.2
MenHibrix (GlaxoSmithKline, Belgium) is a combined Haemophilus influenzae type b–N. meningitidis serogroups C and Y-tetanus toxoid conjugate vaccine (HibMenCY-TT) indicated in the United States for vaccination of infants as a 4-dose series from 6 weeks through 18 months of age. HibMenCY-TT is designed to protect against 3 major causes of invasive bacterial infection in the United States, avoiding additional injections and office visits.3,4 HibMenCY-TT was immunogenic and had an acceptable safety profile when administered as a 4-dose series in phase 2 and phase 3 studies in infants.5–10
Circulating antibody is important in providing long-term protection against invasive meningococcal disease.11 Therefore, evaluating the persistence of antibody responses is critical to understanding the expected duration of protection and the possible need for booster doses. Here we report an extension of a phase 2 proof-of-concept study conducted in the United States, in which antibody persistence was evaluated for up to 5 years after a fourth dose of HibMenCY-TT administered at 12–15 months of age, as well as after a single dose given at that age after priming with Hib-TT alone.
Study Design and Subjects
This phase 2, open, controlled multicenter study was conducted by 12 investigators at 20 centers in the United States. The study was a serological follow-up of subjects previously randomized to vaccination at 2, 4 and 6 months of age with HibMenCY-TT or Hib-TT (ActHIB, Sanofi Pasteur, Swiftwater, PA) coadministered with several other recommended vaccines (101858 clinical trials.gov NCT00129129).5 At 12–15 months of age (102015/NCT00129129), HibMenCY-TT vaccinees received a fourth dose of HibMenCY-TT (HibMenCY x 4 group), whereas Hib-TT vaccinees were rerandomized (1:1) to receive either HibMenCY-TT (HibMenCY x 1 group) or Hib-TT (Hib group).6 All vaccines were coadministered with Pediarix (GlaxoSmithKline, Belgium) and Prevnar (Pfizer) at 2, 4 and 6 months, and with Prevnar at 12–15 months of age, according to the US schedule in place at the time of study conduct. Immunogenicity and reactogenicity of the coadministered vaccines have been reported previously.12 Antibody persistence was reported until 1 year after the fourth dose (107824/NCT00359983).6 This report describes results from serological follow-up at 3 and 5 years after the fourth dose given at 12–15 months of age (107826 [3-year persistence study] and 107829 [5-year persistence study], www.clinicaltrials.gov NCT00359983).
Participants were eligible to participate in the follow-up 3 years after the fourth dose if they were between 44 and 60 months of age, and at the 5-year follow-up if they had been vaccinated 5 years ±8 weeks previously. Participants were required to be in good health, have previously received all 4 study vaccine doses and to have completed the 6-month safety follow-up after the fourth dose. Subjects could not participate if they had received additional Hib or meningococcal serogroups C or Y vaccines, or if they had a history of disease due to H. influenzae or N. meningitidis serogroups C or Y.
The coprimary objectives of the follow-up studies were to evaluate long-term antibody persistence in the HibMenCY x 4 group versus the Hib group in terms of antipolyribosylribitol phosphate (anti-PRP) concentrations ≥0.15 µg/mL; to evaluate the long-term persistence of human complement serum bactericidal activity (hSBA) titers ≥1:8 for serogroups C and Y in the HibMenCY x 4 group and to evaluate long-term antibody persistence in the HibMenCY x 1 group in terms of anti-PRP concentrations ≥0.15 µg/mL and hSBA ≥1:8 for serogroups C and Y.
Protocols were reviewed and approved by appropriate institutional review boards. Written informed consent was obtained from parents or guardians at each follow-up time point.
Assessment of Immunogenicity
Blood samples collected 1 month and 1, 3 and 5 years after the fourth dose were tested for antibodies against PRP and for hSBA against meningococcal serogroups C and Y at GlaxoSmithKline’s laboratories, as previously described.5,13 For anti-PRP, a 0.15 µg/mL cutoff, which is considered a short-term correlate of seroprotection,14,15 was used, and a cutoff of 1.0 µg/mL was also assessed. For hSBA, a conservative antibody titer cutoff of 1:8 was used. However, a cutoff of 1:4 is considered a correlate of protection for serogroup C,16 and it is common practice to extend to serogroup Y. Samples at year 5 were tested using an optimized hSBA-MenC assay; the hSBA-MenY assay was unchanged. Minor adjustments were made to the hSBA-MenC assay procedure in terms of diluent formulation, longer preincubation period, sequential addition of complement and shorter killing time. No change was made to the complement source or the working strain. A bridging analysis showed high concordance in the percentage of subjects reaching a titer ≥1:8 using the old and optimized assays (98% concordance, correlation coefficient [R2] = 0.95). A subset of the year 1 samples from the according-to-protocol (ATP) cohort was retested with the optimized assay. No increase in geometric mean titers (GMTs) was observed; by contrast, GMTs were lower using the optimized assay. Therefore, use of the optimized assay at year 5 was not considered to have resulted in an overestimation of the titers compared with previous years. Tables, Supplemental Digital Content 1 and 2, http://links.lww.com/INF/B442 and http://links.lww.com/INF/B443, provide the results of retesting and assay concordance results, respectively.
Assessment of Safety
During the follow-up period, investigators were asked to report any serious adverse event that in his/her medical judgment could be reasonably related to the previously administered study vaccines.
The immunogenicity analysis was done on the ATP cohorts assigned at each postvaccination and persistence time point. Results from the fourth dose vaccination phase (ATP persistence cohort after primary vaccination and fourth dose ATP immunogenicity cohort) and the ATP persistence cohort for the year 1 follow-up time point are presented here for completeness.6 The ATP cohorts comprised all subjects who complied with protocol-defined procedures for whom immunogenicity results were available for at least 1 antigen, and who were not immunocompromised or had received any blood products at the time of the follow-up visit.
Exploratory analyses compared seroprotection rates and geometric mean concentrations or titers (GMC or GMT) between groups 3 and 5 years after the fourth dose. GMC or GMT ratios were computed using an ANOVA model on the log10-transformed concentration or titer using the vaccine group as covariate. For differences in percentages above thresholds, a statistically significant difference was concluded if the 95% confidence interval for the group difference excluded the value 0; for statistically significant differences in GMC or GMT ratios, the 95% confidence interval for the group ratio excluded the value 1. There was no adjustment for multiple testing, which means that it is possible that a statistically significant result could have occurred by chance alone. Therefore, statistically significantly differences should be interpreted cautiously.
A generalized linear model was fitted to estimate the potential effect of dropouts on GMC and GMT values over the years. The logarithm of the titer was modeled given the time point (defined as a categorical variable in order not to assume any profile of decay), the treatment group and their interaction via a mixed linear model assuming that data is repeated within the same subject along the several time points and without imposing any restriction in the covariance structure of errors (“unstructured” covariance structure).
Statistical analyses were performed using the SAS software version 9.2 (SAS Institute Inc., Cary, NC) and ProcStatXact 8.1 (Cytel Software Corporation, Cambridge, MA).
The study began (first dose given) on August 13, 2004, and the assessment of antibody persistence was completed on May 12, 2011. Of 498 subjects who received a fourth vaccine dose, 270 participated at year 1, 201 participated at year 3 and 181 at year 5 (Fig. 1). There were fewer males in the Hib group at each time point (Table 1). Other demographic characteristics at each time point were similar.
Persistence of Antibodies Against Hib
Five years after the fourth dose, the percentage of subjects with anti-PRP concentrations ≥0.15 μg/mL was 98.8% in the HibMenCY x 4 group, 97.3% in the HibMenCY x 1 group and 92.3% in the Hib group (Fig. 2A). The percentage of subjects with anti-PRP concentrations ≥1.0 μg/mL was 56.0% in the HibMenCY x 4 group, 27.0% in the HibMenCY x 1 group and 43.6% in the Hib group (Fig. 2B). Anti-PRP GMCs decreased over time in all groups: from 28.6 μg/mL after dose 4 to 1.2 μg/mL at year 5 in the HibMenCY x 4; from 10.7 μg/mL to 0.7 μg/mL in the HibMenCY x 1 group and from 19.0 μg/mL to 0.8 μg/mL in the Hib group. By year 5, GMCs were similar across the 3 groups and approached levels observed before the fourth dose at 12–15 months of age (Fig.2C).
Exploratory comparisons did not indicate statistically significant differences between either of the Hib-MenCY-TT groups and the Hib control group in terms of the percentage of subjects with anti-PRP concentrations ≥0.15 μg/mL or in anti-PRP GMCs 3 or 5 years after the fourth dose.
Persistence of Serum Bactericidal Antibodies Against Meningococcal Serogroup C and Y
The percentage of subjects with serogroup C hSBA titers ≥1:8 declined in the HibMenCY x 4 and HibMenCY x 1 groups over time until 3 years after the fourth dose (Fig. 3A). Between 3 and 5 years after the fourth dose, there was an increase in the observed point estimate of subjects with serogroup C hSBA titers ≥1:8 in the HibMenCY x 1 group from 57.1% to 73.5%, whereas in the HibMenCY x 4 group the change was from 81.4% to 82.9% and in the Hib group from 11.1% to 21.1%. In all groups the magnitude of the hSBA GMT for serogroup C increased from year 3 to year 5 but was most marked in the HibMenCY x 1 group (from 16.0 to 44.2 versus 41.9 to 69.8 in the HibMenCY x 4 group and from 2.7 to 3.6 in the Hib group, Fig. 3B). No formal statistical comparison was performed because the control group did not receive a meningococcal vaccination during the primary study
GMT estimates using the generalized linear model were found to be similar to observed values (data not shown), indicating that the observed results were not likely to be significantly biased due to subject dropouts.
Five years after the fourth dose, 69.5% of subjects in the HibMenCY x 4 group and 54.3% in the HibMenCY x 1 group continued to have hSBA titers ≥1:8 for serogroup Y. The percentage of subjects with hSBA titers ≥1:8 and hSBA GMTs declined over time, but at year 5 remained higher than the percentages observed before the fourth dose at 12–15 months of age (Fig. 2C and D).
Three years after the fourth dose, exploratory analyses suggested that the percentage of subjects with hSBA titers ≥1:8 and the GMT were higher in the HibMenCY x 4 group than the HibMenCY x 1 group for serogroup C. Exploratory analyses indicated no statistically significant differences between the HibMenCY x 4 group or the HibMenCY x 1 group at year 5 in terms of the percentage of subjects with hSBA ≥1:8 or GMTs for serogroup C or Y.
For the HibMenCY-TT x 1 group, the reverse cumulative curve for hSBA-MenC at year 5 seemed to be shifted to the right compared with the one at year 3 (Fig. 4). This would tend to indicate that the increase in titers between year 3 and year 5 may have occurred for all subjects regardless of their year 3 titer value. However, these results should be considered cautiously given the small number of subjects in each group and the fact that distributions and not individual values are being compared.
No potentially vaccinated-related serious adverse events were reported during the persistence phase of the study.
Five years after a fourth dose of HibMenCY-TT administered at 12–15 months of age, the majority of recipients retained seroprotective levels of antibodies to all 3 vaccine antigens. The data indicate that use of HibMenCY-TT results in comparable anti-PRP antibody persistence to a US-licensed Hib-TT vaccine. Moreover, 5 years after vaccination, 82.9% of subjects who had received 4 HibMenCY-TT doses had hSBA titers ≥1:8 for serogroup C, and 69.5% had hSBA titers ≥1:8 for serogroup Y.
Some subjects in the Hib group (who were never vaccinated with meningococcal vaccine antigens) developed natural antibodies against serogroup C and Y over time, which is consistent with the gradual acquisition of natural immunity during childhood.16,17 However, the percentage of subjects with hSBA levels associated with protection was well below that observed in subjects 5 years after vaccination with 4 doses of HibMenCY-TT doses or a single dose of HibMenCY-TT at 12–15 months of age.
We observed a possible increase in serogroup C hSBA responses between year 3 and 5 in subjects who had received a single HibMenCY-TT dose at 12–15 months of age after Hib priming, although it should be noted that the 95% CIs between year 3 and year 5 overlap. Given the good correlation observed between the optimized assay used at year 5 with the assay used previously, we consider it unlikely that the minor changes in assay procedure at year 5 contributed to this result. Furthermore, because all groups were tested with the optimized assay at year 5, similar findings would have been expected in all, not just one, of the study groups if the increase was assay related. It is possible that subjects in the HibMenCY-TT x 1 group with low titers before exposure were more susceptible to natural boosting than the subjects in the HibMenCY-TT x 4 group, who had higher titers before the fourth dose. However, the data from this study cannot be used to confirm this hypothesis. In fact, this study is potentially limited by the small sample size arising from a high number of dropouts over time, with only 36% of subjects who received the fourth dose returning 5 years later.
Future studies are planned to assess antibody responses after each vaccine dose and will evaluate coadministration with recently licensed and routinely recommended pediatric vaccines, including 13-valent pneumococcal conjugate vaccine, rotavirus and hepatitis A vaccines.
In conclusion, within the limits of the study, HibMenCY-TT given as a 4-dose series at 2, 4, 6 and 12–15 months of age, or a single dose at 12–15 months of age, induced protective immunity against all 3 vaccine antigens that persisted for up to 5 years after vaccination in more than half of the subjects.
MenHibrix and Pediarix are trademarks of the GlaxoSmithKline group of companies. Prevnar is a trademark of Pfizer.
The authors thank the volunteers and their parents/guardians who participated in the study, as well as the study nurses and other staff members, in particular the following investigators who participated in the study: Shelly Senders, MD, Shelly Senders & Associates, Inc, OH; Stephen Rinderknecht, DO, Lakeview Pediatrics, IA; Thomas Latiolais, MD, Ark–La–Tex Children’s Clinic, LA; Wilson P. Andrews, MD, Pediatrics & Adolescent Med PA, GA; Malcolm Sperling, MD, Edinger Medical Group, CA; James A. Hedrick, MD, Kentucky Pediatric/Adult Research, KY; Scott Sanders, MD, Arkansas Pediatric Clinic, AR. The authors also thank Leentje Moerman for study report writing, Yaela Baine for input in data interpretation, Magalie Caubet for statistical analysis (all from the GlaxoSmithKline group of companies), Dr. Joanne Wolter (independent medical writer) for writing an initial draft of the article and Dr. Wouter Houthoofd (XPE Pharma & Science) for publication coordination, on behalf of GlaxoSmithKline Biologicals SA.
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Haemophilus influenzae type b; meningitis; Neisseria meningitidis serogroup C; Neisseria meningitidis serogroup Y; vaccine; antibody persistence
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