With the aim to confer dual protection against the 2 viruses with a single vaccine, GlaxoSmithKline (GSK) Biologicals developed a combined vaccine against hepatitis A and B (HAB; Twinrix), which was made available in 1996. This vaccine is available in 2 formulations administered at 0, 1, and 6 months schedule: adult (720 EL.U of hepatitis A antigen [HAV] and 20 μg of hepatitis B surface antigen [HBsAg]) for those aged ≥16 years, and pediatric (360 EL.U of HAV and 10 μg of HBsAg) for those up to 15 years of age. The adult formulation is also licensed for use as a 2-dose schedule (0, 6 months) (Ambirix) in children and adolescents from 1 to 15 years of age in many countries.
The 2-dose regimen of the adult formulation was found to have a good immunogenicity and safety profile in children and adolescents1,2 and has a similar safety and immunogenicity profile to the well-established 3-dose regimen of the pediatric formulation in children1,3 and adolescents.4,5
The present study evaluated the long-term (10 years) persistence of antibodies against hepatitis A and B (anti-HAV and anti-HBs, respectively) in children aged 1 to 11 years at the time they received 2 doses of the adult formulation of the HAB vaccine, in the primary study.6 Anti-HAV and anti-HBs immune memory in terms of anamnestic response to a challenge dose in those who became seronegative for anti-HAV antibodies (ie, anti-HAV antibody concentrations <15 mIU/mL) or had anti-HBs antibody concentrations <10 mIU/mL was also evaluated.
MATERIALS AND METHODS
Study Design and Subjects.
The open-label single group primary study was conducted at 1 center in Belgium in 1998. Healthy subjects aged 1 to 11 years, without history of hepatitis or vaccination against HAB, received 2 doses of the HAB vaccine containing 720 EL.U HAV and 20 μg HBsAg, according to a 0, 6 months schedule. These subjects were then followed up for the next 10 years, with the blood samples collected at yearly visits (NCT00289744). Subjects were eliminated from the long-term According-To-Protocol (ATP) cohort for immunogenicity analyses if serology results were missing, if they had received an additional dose of monovalent or combined HAB vaccine, or if they demonstrated an unexpected increase in antibody titers.
All subjects with anti-HBs concentrations <10 mIU/mL at the follow-up visits between year-6 and -10 received a challenge dose of a monovalent hepatitis B vaccine (HBV; Engerix-B, GSK Biologicals, Belgium) 10 μg anti-HBsAg (pediatric formulation) for subjects aged ≤15 years and 20 μg (adult formulation) for subjects ≥16 years]. The vaccine was administered as a deep intramuscular injection in the deltoid. For these subjects, additional serum samples were collected before and 1 month after challenge vaccination.
The study was conducted as per the Good Clinical Practice guidelines and the Declaration of Helsinki, with necessary approvals from the Ethic Review Committees, after written informed consent was obtained from subjects/parents/guardians.
Assessment of Immunogenicity.
The anti-HAV antibody concentrations were measured throughout the study using a commercial enzyme immunoassay (Enzygnost/DADE Behring; cut-off: ≥15 mIU/mL), at GSK Biologicals laboratory. The anti-HBs antibody concentrations up to the year 8 were measured using a commercial enzyme immunoassay (AUSAB EIA/Abbott; cut-off: ≥3.3 mIU/mL) at GSK Biologicals laboratory, while for year 9 and 10, an equivalent validated in-house enzyme-linked immunosorbent assay with the same cut-off was used at Centre for Vaccinology, Ghent University and Hospital.7
Assessment of Safety.
For the challenge dose, solicited local and general symptoms occurring during the 4-day postvaccination follow-up period and unsolicited adverse events occurring during the 31-day postvaccination follow-up period were recorded. Serious adverse events (SAEs) occurring during the entire follow-up period were recorded.
Descriptive immunogenicity analyses for the long-term follow-up phase were performed on the long-term ATP cohort for immunogenicity.
The immunogenicity analyses for the challenge phase were performed on all subjects who received the challenge dose. Anti-HBs anamnestic response to the challenge dose was defined as postchallenge dose anti-HBs antibody concentrations ≥10 mIU/mL in subjects who were seronegative (<3.3 mIU/mL) prechallenge dose or at least a 4-fold increase in postchallenge dose anti-HBs antibody concentrations in subjects who were seropositive (≥3.3 mIU/mL) prechallenge dose. The safety analyses for the postchallenge dose were performed on the total vaccinated cohort.
To assess the comparability of the year-10 long-term ATP cohort and the originally vaccinated cohort in the primary study, post hoc statistical analyses were conducted to obtain data on age distribution, gender ratio at enrollment, and immune response to primary vaccination (post-month 7) in subjects who were eventually included in the year-10 long-term ATP cohort.
All statistical analyses were performed using SAS version 9.1 (SAS Institute, Inc., Cary, NC).
A total of 232 subjects completed the primary study. The year-10 long-term ATP cohort (N = 120), although comparatively smaller, was a true representative of the overall study cohort in the primary study (N = 237), as evident from the similarity between the 2 cohorts in terms of age and gender distribution at enrollment (given in the demographic section) as well as immune response following the 2 primary vaccine doses post-month 7 (anti-HAV seropositivity rates [100% and 100%, respectively] and GMCs [12,214.8 mIU/mL and 11,543 mIU/mL, respectively] and anti-HBs seroprotection rates [99.2% and 98.5%, respectively] and GMCs [9614.3 mIU/mL and 8056 mIU/mL, respectively]).
Ten years after primary vaccination, the mean age of subjects was 17 years (standard deviation = 2.84 years) and 54.2% (65/120) of the cohort was females. Post hoc analyses indicated similarity in age distribution (ratio of subjects aged <6 years and ≥6 years: 39/81 [0.48]; 77/160 [0.48], respectively) and gender distribution (percentage of males: 45.8% and 47.3%, respectively) at enrollment, between the year-10 long-term ATP cohort and the originally vaccinated cohort in the primary study.
One month after the second vaccine dose, all subjects had seroconverted for anti-HAV antibodies, whereas 98.5% of subjects were seroprotected for anti-HBs antibodies. Ten years after primary vaccination, all subjects continued to remain seropositive for anti-HAV antibodies and 81.7% of subjects continued to have anti-HBs antibody concentrations ≥10 mIU/mL (Tables 1 and 2).
Of the 38 subjects who were eligible to receive a challenge dose of monovalent HBV vaccine (anti-HBs antibody concentration <10 mIU/mL), 13 did not receive the vaccine (due to consent withdrawal: 6; could not be contacted before vaccination: 2; lost to follow-up: 4, death: 1). Of the 25 subjects who received the challenge dose, 6 subjects (aged ≤15 years) received a 10-μg dose, while 19 subjects (aged ≥16 years) received a 20-μg dose. One month after the challenge dose, all subjects had anti-HBs antibody concentrations ≥10 mIU/mL and the corresponding GMCs in those receiving the 10 and 20 μg doses increased markedly from 8.7 to 565.9 mIU/mL (65-fold) and from 10.4 to 1431.9 mIU/mL (138-fold), respectively. An anamnestic response to the challenge dose was observed in all subjects irrespective of their prechallenge dose serostatus (seropositive [≥3.3 mIU/mL]: 24 subjects; seronegative: 1 subject) (data not shown).
Safety and Reactogenicity.
No vaccine-related SAEs were reported during the 10 years after primary vaccination.
The additional HBV dose was well-tolerated in both age groups. Pain at injection site and fatigue were the most frequently reported solicited local and general symptoms (36% and 20% of subjects, respectively) during the 4-day postchallenge dose follow-up period. One unsolicited adverse event was reported, which was considered by the investigator to be causally related to vaccination (syncope in 1 adult on the day of vaccination; resolved on the same day). No SAEs related to the additional HBV dose were reported.
This study is the longest follow-up of the 2-dose adult formulation of the HAB vaccine to be reported in healthy children. A previous 5-year follow-up study in children aged 1 to 11 years has reported comparable long-term persistence of anti-HAV and anti-HBV antibodies, following a 2-dose adult formulation versus 3 doses of pediatric formulation of the same vaccine.8
Ten years after administration of 2 doses of this vaccine, all subjects remained seropositive for anti-HAV antibodies (≥15 mIU/mL), while 81.7% of subjects continued to have anti-HBs antibody concentrations ≥10 mIU/mL. The present study reaffirms the observations of long-term persistence of anti-HAV and anti-HBs antibodies from previous studies with 2- or 3-dose schedules of the adult formulation of the HAB vaccine in adolescents9 and adults.10
It is believed that a decrease in anti-HBs antibody concentrations over a period of time does not necessarily indicate loss of protection. In the present study, immune memory 10 years after the primary vaccination course was shown through mounting of an anamnestic response to the challenge dose of HBV vaccine in all subjects whose antibody concentrations were <10 mIU/mL.
In conclusion, the 2-dose regimen of the adult formulation of the HAB vaccine is highly immunogenic and confers long-term persistence of vaccine-induced anti-HAV and anti-HBs in healthy children. This regimen with a reduced number of injections and clinic visits may lead to greater acceptance of the vaccine.
The authors thank Priya Diana Crasta for performing statistical analysis; Avishek Pal for medical writing assistance; and Manjula K. for publication coordination. All are employed by GSK Biologicals.
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Keywords:© 2011 by Lippincott Williams & Wilkins, Inc.
children; combined; HAB; long-term; two-dose